Gameplay mechanics themselves are fair game and can’t be copyrighted anyway. And I do see the point, and I like that idea too. My idea came from my experiences in Diablo 3, and the fact that me and my friends could keep the boss mostly on lockdown without eliminating its ability to kill us in a second. I feel this opens the door to do something other than “punch the monster, dodge the monster, win.” If balanced correctly, the idea of locking down a boss could be something that happens extremely rarely and ultimately not last very long, and in many cases it might not even be the ideal way to win. I think the longest I’ve ever locked down a boss was 8 seconds by spamming tons of frozen, and in order to be able to do that I had to sacrifice half my damage overall compared to other builds.
As far as constant interrupts, that’s why I suggested timed immunity as their defiant stacks rise. It’s also not even a little far-fetched to have the boss be immune to stun/interrupt during some or many special attacks. Look at world of warcraft bosses (I know, I know, just hear me out): Many of guild wars 2 gold have devastating attacks or effects that can put the party in severe danger, but are also given a chance to be interrupted without requiring some kind of gimmick from the dungeon. Aside from those, though, they have plenty of other nasty skills that need to be avoided and cannot be interrupted. Players can often tell these attacks apart because the boss has tells and visual cues indicating whether they can be interrupted or not.
No matter what you change about a system, there are always workarounds to prevent things from being abused while still improving the depth of an encounter.