It's not on the same scale or in back to back episodes. It does take a toll on the actors. There's not a regular character who is in constant full head makeup for the entire series.
It probably could be done, there just wasn't the budget to allow it. Although some of the behind the scenes material mentions how much 3D printing has sped up the process and allowed them to make more unique pieces. Old Trek was limited by the budget and the reality of TV production at the time. They probably had a few months to a few weeks to design, mold and use an alien makeup or new character.
Conversely you did t need HD detail. And yeah Buffy didn’t have a constant cast member in makeup, but I strongly suggest Westmore could have done it on Trek.
The reality is the redesign is a redesign for the sake of it...Klingons could have looked like this back then, and the Klingons now could look like they did then now. The tech isn’t radically different.
Silicone and some of the newer variants is about the only radical leap in makeup FX. That and everyone being more interested in it.
I am sure Neville would talk about the advances in design from Z Brush, but really, he’s shown that can be outperformed by a dude or dudette with some clay on face off. That tech and 3D printing are just a divergence, originally for taking trad work into games and such like...working the other way is almost counter intuitive.