It's a regressive change.

There were people who considered themselves to be 'main tanks' in FFXIV. That was circa 2013, and they insisted on stacking max parry and VIT. Meanwhile, their skilled colleagues were maximising STR and damage while using using coordinated mitigation and swaps with their co-tank smartly to offset the defensive difference. Gordias ended that debate years ago (it continued on this subforum until about Stormblood, but not everyone got the memo). A more modern game probably wouldn't even implement a trinity design, let alone designate a 'main tank'.

I think the only potentially interesting change here is the idea of counterattacks. However, the planned implementation is something that has been tried and has failed before (because players find ways to take more damage to get more procs). I would be much more impressed if they reworked interject/stun to have job specific flavour and gave all tanks the ability to give mobs vulnerability in exchange selectively countering specific scripted attacks (especially if they're replacing the current system of timed raid buffs). That's the type of counterattack that I'd like to see, rather than spamming Shield Swipe procs off of every raidwide.