Weapon damage is irrelevant. Potency is the only value you need to look at to see how much damage two characters would deal when comparing rotations.
Let's take a look at WAR. We have a level 30 WAR & a level 80 WAR. We'll both give them all level 1 gear & level 1 weapon so when inside, their stats are almost identical and weapon damage between them is the same, so the only thing that matters is potency and nothing else.
Billy the level 30 WAR rushes in, pops berserk, does 5x overpower, plus 4x normal Overpower outside of berserk for good measure. Just for simplification, we'll make crits do 1.5x damage for damage.
Billy does (130 * 1.5(crit) * 1.2(DH)) = 234 potency for each of his initial 5 overpowers in Berserk, for a total of 1170 potency. his extra 4 overpowers bump it up to 1690 potency. Not bad Billy.
Chad the level 80 WAR rolls in and has 100 beast Gauge, 2x Infuriates and IR ready. He rolls in and pops IR, followed by 5x Decimate. He does (250 * 1.5 * 1.2) = 450 potency per hit. With his 5 decimates, he does 2250 damage. Already nearly 1.5 times as much damage as Billy did. But we're not even done yet.
During that IR, Chad got to use a free upheaval (450 potency) & Onslaught (100), meaning Chad has now done 2800 potency total, now closing in on nearly 2x as much as Billy. Chad does 2x more Decimate to drain his 100 beast gauge, adding another 500 potency, now up to 3300 potency. Chad now finally whips out his 2x Infuriates, doing 2x Chaotic cyclones. (400 * 1.5 * 1.2) = 720 potency, or 1440 potency total. Meaning Chad has now done 4740 potency, or nearly 3x as much damage as Billy.
But wait! Chad also has Storm's eye, giving him a 10% damage boost! meaning he actually has done more than 3x as much damage than Billy, at 5214 potency. He did over 3x as much damage as Billy in the same 9 GCDs as Billy, and this is only on AoE (Spoiler alert: Chad is doing over 5x as much damage as Billy between Fell Cleave & Inner Chaos on single target). We haven't even talked about the fact that while Billy is bleeding out from damage over there, Chad is popping his Nascent Flash to completely laugh at his enemy's attempts to hurt him by life-stealing himself to full HP from just a single decimate!
Even outside of burst, Chad can do nearly 4x overpower's worth of damage every 30s compared to Billy from Upheaval. Chad can also do 70 more potency every 2 GCDs, and every 4-6 can do 2x Overpower damage in the form of Decimate! Which not only deals more damage, but also reduces his Infuriate Cooldown closer to Wreck Billy's damage harder when he can use more Chaotic Cyclones! And what does Billy have to answer to Chad? nothing.
I think you don't understand that weapon damage will do nothing. Given equal gear to nullify weapon damage out of the equation, a level 80 will still be doing several times more damage than a level 30 could ever possibly do. This is also just for Warrior, a job that is largely GCD bound. I actually tried NIN first but that was getting so hilarious at how hard a level 80 NIN was curbstomping a level 30 one that I switched to Warrior as a more tame example.
I also enjoy how you brought up a slippery slope earlier when you're the one talking about how we should just completely blow out balance just because newer jobs weren't as properly tuned to low levels as they should have been. The solution is to fix those jobs at lower levels, not let every job do the same.
Level Sync will never change. It's been brought up countless times in the past, the devs have mentioned it in liveletters why they keep it as it is. it's to maintain parity between a level capped character of a job and a low level character of the same job. When the current system is future proofed for every expansion to come, requires zero tweaking of potency, %s, etc to work, and does a dang good job at keeping two characters of differing levels in a job to maintain parity, why would they ever spend even a nanosecond designing a new system? "Don't fix whats not broken."
As far as your worries, the roulettes already provide the highest exp-time ratio the game offers. and if by some unholy miracle people decide leveling roulette isn't giving enough and mass exodus? They'll just crank the rewards up ala MSQ roulette. Put on your favorite streaming service, kick back and relax with your 2 button rotation, or simply avoid roulettes altogether.