I've always found it frustrating that TTK received almost universal praise and was widely regarded as a major improvement to the formula when (in my mind) it completely ruined the game. Sure, some minor QoL improvements were added as well as a handful of new features that were certainly welcome, but the core of the game (as you mentioned) was destroyed. As far as vanilla being the best time in D1's lifespan, I think the HoW era is more deserving of that title, simply due to the addition of Etheric Light. That's not to say vanilla wasn't fun, however. On the contrary, it had (in my opinion) the most enjoyable Crucible meta and the best selection of weapons and gear. That being said, I still consider HoW slightly superior since it offered players an unprecedented level of choice by making old content relevant again.