I suspect a lot of the healers striking are already donuts to begin with. You yourself, the creator of this thread, have admitted to not doing much higher-end content. Frankly, your opinion is completely irrelevant because of that. You'd see that roles actually matter more in harder content if you bothered to do it. Sure, ucob was cleared with 8 tanks. Sure, top was cleared without any healers. Sure, tea was cleared with 2 tanks, 5 white mages and a dragoon. But that's not the norm. Nobody is actually doing that because it's optimal, they're doing it for fun. To see if they can actually do it.
Casual content is casual content. For better or worse, it's meant to be cleared by anybody at any skill level. This is unfortunate in my opinion because people can get to max level and still have exactly 0 idea what they're doing. Just look at most mentors for proof of this. Warriors, in dungeons, are awesome. They can do the entire thing no problem. Warriors, in trials, are still awesome, but only to a certain extent. Warriors, in normal raids, are still awesome, but only to a certain extent. When you get up to the extreme level, the savage level, the ultimate level, the sustain of tanks (and dps) matters less and less because the fights are mechanically difficult, not healing difficult. Healers matter most, in my opinion, in prog, when everybody's still learning and making mistakes. That part is where healers can shine with their improvisation skills.
Content in this game is a puzzle. It might be a difficult puzzle to figure out, but once you do, that's it. It's solved. This means that once you've got it down, tanks will always know where they're putting their mits, healers will always know where they're putting their heals, dps will always know where they're putting their mits, when they're using their burst, etc. etc. That's just how it is. So instead of making healer good, or making tanks worse, there would need to be a complete overhaul of the battle system to make it less puzzle-like. I'm far from the first person to have this idea. I genuinely don't think this is ever going to happen. Again, for better or for worse, the game's gotten easier over the years and it's now "acceptable" now more than ever to be bad.