I personally dislike it. It kind of kills conscious decisions for me because of the fact you have to put in 900 mass in order to get more mass with a t2 mex. If you do it in little steps, people would be "oh, I have no clue what to do and I float a little mass, I guess it's time to just assist a random mex for a bit" instead of thinking more actively whether units are needed / wanted or if they can afford a t2 mex.
Not to mention it'd increase the difficulty a lot if it comes to reading enemy bases (if I upgrade all mexes to t2 but pause at 90%, either you look closely when scouting or you'd have literally no idea since they don't get gray'ed out) or the game in general.
So overall interesting idea, could be a mod, but for me personally way too many downsides