All said and done: When you keep your engine moving forward there is only so much legacy support you can offer and it seems we're gradually reaching the point where old models just can't and won't work anymore. I'm sorry if you preferred the old ones, truly I am, but nothing short of breaking the ToS/EULA will allow you to use them now and even that might not work.
That's the end of it. I appreciate it sucks and I'm sure the old models were very personally dear to you. Unfortunately games either keep moving forward or they stay still and rot. Vanilla will be here eventually for you.
It's not that they can't or won't work, Blizzard purposely decided to just go "fuck it" and outright remove them instead of keeping them.
Not something I'm a fan of either because UD male models, SINCE LEGION PREPATCH, has bugged as fuck shoulders that are seated wrong. Had to racechange all of my characters due to it.
If you have been using the old models since Legion then you have not had access to many of the new animations and effects added since then. Those new additions work exclusively with the animation engine developed after MoP launch and were never retrofitted. The suggestion that Blizzard should develop matching animations for legacy models is unfeasible. It'd literally double-or-more the workload, that's if the original animation tools even still exists and haven't been cannibalised by the tools team.
FOMO: "Fear Of Missing Out", also commonly known as people with a mental issue of managing time and activities, many expecting others to fit into their schedule so they don't miss out on things to come. If FOMO becomes a problem for you, do seek help, it can be a very unhealthy lifestyle..
The only thing I can think of is that it has something to do with highmountain tauren. Currently the only races that can't use the old models are Draenei, Night Elves, Blood elves, and Tauren, all of which share a skeleton with an allied race that's implemented in 7.3.5.
I agree that if they intend to give people the option they shouldn't randomly remove it.
But I'm curious: why would you want to run through 2017-2018 updated zones, with hi-res mounts and hunter pets, and sometimes even gear, on a character from 2004 with a quarter (or less) of the polygon count and a texel density that is laughable by today's standard?
Yeah, it sucks, but it was inevitable
I am the one who knocks ... because I need your permission to enter.