I did read your linked post, and you're literally offering nothing new to the table with that idea. We already have cross-class skills you can't equip when you change to a job, but you can use as a class without a soul stone should you decide to go for utility over specialization. Look at that: you have a choice! And there's people on this forum who even forego the soul stone so they can use more skills for said utility and they can get through content just fine.
And outside of raids (which is content we've both said we won't do) those cross-class skills are 100% optional. No one's forcing you to use them. They weren't designed for your class. But there's a choice to customize your character's rotation with them.
I constantly see people either ignore or flat out refuse to use cross-class skills because "I'm a mage, I shouldn't use archer skills" or whatever. Fine, that's your choice. But if you're going to turn right around and complain that you have no customization available to you then you're ignoring what the game already offers just because it doesn't neatly fit into your labels, then you only have yourself to blame.
Now the rest of this wall of text isn't directed at you since I know you're in favor of cross-class skills. This is more towards people who want skill trees, specs, or just control over attributes.
I can only speak for BLM since that's the class who's level I have highest, but I have a ton of options to personalize my rotation and still be effective. Which is why I actually get a little offended that even other BLMs say we only have fire and ice. If you're only using fire and ice then you're the one pigeon-holing yourself.
Raging and Quelling Strikes aren't required skills by what I see in DF. As only one other BLM I've ran into in DF actually used either one. Raging especially you don't have to use now that we've got Enochian. But I can choose to still have that skill on my hotbar because whenever Enochian's down, I can pop Raging instead depending on the stance I'm in whenever Enochian is gone. Some people find that redundant and would rather swap Raging for something else. Quelling strikes in theory isn't required because I've seen most tanks be able to handle BLM enmity magnets without it in place. But if you choose to have it there, it can take some stress off the tank or make up in case your DPS-ing partner isn't attacking the right person (or you on accident aren't, you can pop that to help mitigate the mess that's been made).
I don't have to have Physick/Psycik on my hotbars because my healing ability is utter trash compared to healing roles. But for those that choose to use it, it can actually pull you out of a bind. I can't count how many times I've had to drop my role as DPS, go into Umbral Ice, and start healing because the healer died for whatever reason. Or the healer's taken damage in a boss fight but they can't afford to take attention off the tank. Pop a few heals on them while I'm regenerating MP, they don't even have to waste cooldowns or AoE healing on members that didn't need it. They could keep doing what they're doing.
I don't personally use Virus because I don't know fights as well as others, but I've seen a BLM who did to great effect and it helped the tank. We have skills that teleport us to another player to help get out of the bad. Skills to prevent damage to party mates. Etc. SMNs have even better utility because they can Raise. They certainly don't have to, but it is amazing when they do.
Now in games that do skill trees or specs, I do not have the same flexibility in what I can do in a fight as I can here in this game. In WoW as a Warlock, if my healer dies I can't suddenly help with the healing until someone gets that healer back on their feet. I can't resurrect that healer. My single role is DPS and boy does that game want me to do nothing but DPS even if the situation would be more fun/helpful if I could step out of that box for 10 seconds to aid my party.
That's not even all the skills BLM, one job, has as optional skills that we as a class can use. There's plenty of skills in our own native toolkit that are honestly optional/situational that players flat out refuse to even look at. Then they scream the job is boring. Now other classes/jobs might not have that same flexibility through their cross class skills. And if that's honestly true, then I can see why people feel the need for more customization. But given the vast number of players that ignore over half the skills my own class/job can use, I'm sure others in other roles do the exact same.
Skill trees and such pigeon-hole you into doing one job and one job only. At least in this game with cross class abilities should shit hit the fan, I can switch gears on a dime and help my party until we're on our feet again. Name me a game with skill trees or specs that gives me that same variety and I will gladly shut up.