The main point of the self-healing changes it to make tanking comfy. What typically happens with beginner tanks is that they see their HP bar dropping below about 70% or so, and then they start panicking. They then blow through all their cooldowns or start raging at their healer. They also tend to lack an awareness of damage patterns, causing them to react too late and forcing their healer to burn resources to dig them out of the hole they put the team in. On-demand self-healing solves both of these problems. It also makes beginner tanks feel more in control of the situation, now that pretty much anyone can pick up a tank and solo something regardless of skill.

I also think that it makes sense for PLD and WAR to be most invested in self-healing, lore aside. If you load up a game for the first time and choose to pick up a sword or an axe, these are the two jobs you'll probably end up playing as your gateway into tanking. It's a much longer investment to complete through to Heavensward, jump into the Brume in Ishgard, and start levelling DRK back up from 30, and if you're doing all that, you've probably already know what you want to get into.

If self-sustain was intended to be a skilled endeavor, it would be timing dependent (i.e. press button to reverse damage received over the past 2 seconds, up to X% of your total HP.) That puts you into a situation where you still have to understand the damage pattern, and you're less likely to recover from a slip up. If DRK was to go the self-healing route, that's what I would personally prefer. But I think that the bubbles are equally interesting, and I'm really hoping that they just double down and turn it into a thematic focus. Nothing says that a tank has to have % damage reduction abilities at all. You can do everything with flat damage reduction if you know the damage numbers.

They might tone down some of these numbers depending on how the raid tier is tuned, but like it or not, this is likely a deliberate change to make tanks more accessible for newer players. If you're looking for carry potential in harder content that you won't be able to solo, paradoxically you may have to look elsewhere.