It seems silly that it is more cost efficient to kill a t2 mex and rebuild it as a t3 mex than upgrade.
This does require APM, and hence skill in some form
However I dont feel this makes the game more exciting to play or watch. APM could be better spent microing units.
I think it would make sense to increase the cost of a standalone T3 mex slightly (~ 400 mass) to fix this, and make it more efficient to upgrade existing mex