The thing is that Leveling is actually forever because level sync exists. This is especially true for any player still using the Duty Finder for roulettes. Players don't stop running the Duty Finder even if they finish leveling up because they run roulette for tomestone gear and rewards, which means level syncing down will always be prevalent even at endgame. Therefore, they can still queue up to their main job (ninja in this case) and be level synced down - thus reduced to the experience of "leveling".
Level syncing down will mean the leveling process will become more common than any - especially for any new player or veterans who haven't done old content and want to experience them as close as possible. I strongly disagree that this is a temporary situation. In content where your level sync is not at the endgame level cap, you will always be playing with a worse toolkit as a result (Palace of the Dead, Heaven on High, Bozja, old Extremes, old Ultimates, etc.). Endgame content is temporary, because the current endgame will always be old endgame by the time new endgame content comes out. However, old endgame content will always exist and in greater amounts - which means leveling will always be forever.
The problem of tuning solo content such that it can be completed by any job is that all jobs can already complete solo content. However, is it fun for healers with their single attack skill to take forever to beat up the enemies because incoming damage is so low that they can't use their healing toolkit fully at all? No. Not at all. This is a core problem of job fantasy when it doesn't align to the job & to the encounter. You wanted to play as a healer but you became relegated to gimped DPS pressing the same attack button over and over until the instance ends. If you can't get the flavor of your job fantasy, no amount of retuning will feel good. The same applies to ninjas with their assassin job fantasy here. It feels like utter garbage to play as an assassin role that is known for going solo, but not have the ability to properly assassinate the enemy because a large portion is just made up from having allies to do it for you.
5.0 ninja was actually very weak. They got buffed heavily in 5.08 5.1 ninja was when it felt strong because Trick Attack overall rDPS got nerfed (10% to 5%) and the overall toolkit had been buffed to compensate. - Aka rebalancing Trick Attack to increase ninja's personal damage.
And that's the thing. Because of how prominent Trick Attack was to raiding, Ninja ended up developing a trick attack identity. People hated ninja when ninja wasn't a trick attack identity. However that's not the only identity that makes up a ninja. It's two core identities were always - support + fast paced high burst DPS via through melee or ranged (due to assassin job fantasy). If Trick Attack became too impactful, ninjas basically put in a ton of work on their core rotation for low DPS. That's why there were plenty of problems with balancing ninja to not just be a "trick attack" job for raiding and to make their own damage toolkit still impactful. I don't play ninja for trick attack only, I play it because I want that assassin job fantasy outside of raiding. I might want to support my teammates sometimes, but I also want to feel impactful when I'm doing solo stuff because this is a job that is supposed to be able to annihilate an enemy instantly when they use their entire burst. And honestly, sometimes it doesn't feel like that. Sometimes I use my entire burst and the enemy's still kicking, hale and hearty. It really puts ninja's job fantasy in a weird place for that reason because of how much Trick Attack makes up for ninja's damage. Raiding should not be the only thing that influences a job identity if the job fantasy doesn't align with the identity.
It's why I think PvP makes ninja exactly that good. You still get lots of support utility (Raiju stun, Doton heavy + Goka Mekkyaku synergy, Huton shield + movement speed buff, Meisui healing buff, damage up via mug) but your utility is all focused on enhancing yourself and becoming more impactful / flexible to adjust to the situation. It captures the essence of the job fantasy without losing any value as a support-based job. The problem is that this can't translate well into PvE due to how static and inflexible job design & encounter design has been.
Dancer and Bard are known for being a support ranged job that supports their allies' DPS in particular, which is why their personal DPS is generally much lower as a result. They have multiple skills designed to be support oriented for a team setting. However, ninja doesn't fall into this category of being a job that solely supports their team. Ninja is the only job that holds two unique identities that counteract each other. In the past, Ninjas focused on support - yes, but that support also aligned to their personal DPS. In the past, they had Slashing debuff and dripping blades. While some jobs can take advantage to their slashing debuff, their debuff solely exists to help them - with Fuma Shuriken being used often over Raiton for clipping as it benefitted from both slashing debuff and dripping blades. Their whole toolkit - from support to damage all amalgamated to give that quick and high burst DPS job fantasy, which is what made it fun to play in a party setting and in solo instances. No matter where, they always had a toolkit that fit their job fantasy.
If you asked me what changed from ARR to Shadowbringers, it's that ninja became more streamlined and removed a lot of personal support buffs to directly increase their overall DPS per skill. Trick Attack was the only support buff that still exist as a result. However, Shadowbringers still kept their core job fantasy of being a high damage assassin via Hyosho Ranryu. Even then, it took some work to adjust ninja to feel powerful. On its initial release, ninja was actually undertuned again when mudras moved over to a GCD-centric gameplay and went through multiple patches and buffs to make it feel good again for leveling - without just trick attack padding.
From Shadowbringers to Endwalker, we just lost flexibility to account for the big DPS gains other jobs have (raiju being lost upon using another weaponskill and Phantom Kamaitachi being an underwhelming skill on release). It honestly felt like they were trying to balance Ninja's rDPS because of how much stronger Trick Attack was, but our overall DPS didn't change much. It was after multiple adjustments on these skills that we got ninja's overall DPS back to where it was again - which is also due to its dual identity as an high burst DPS assassin. On Dragon Song Reprise release though, Trick Attack's damage got moved to mug - which I believe is exactly because of the issue of how impactful their RDPS was in relation to its personal DPS.
I would be fine if there was a job that was more support oriented than personal DPS oriented, but that's not what ninja's job fantasy is. It would be great if it can have both, but it becomes an issue later down the line, I am more willing to move the trick attack's raidwide debuff and have another job be what ninja couldn't if it becomes too much of an issue with its job fantasy.