The key word here is "rush" if someone rushes t2 on the com and makes TML, they will have it before a normal player has upgraded to T2 at a normal rate.
If the TML was more expensive yes you should be able to kill it but as it costs 800 mass almost a T2 mex how much eco is too much eco, 1 single mex?
you give up one T2 mex in order to kill 10.
A TML is worth the same as 14 T1 Strikers which his hardly a game changing amount so if you have t2 on the com and you get rushed you just make t1 pd and easily defend against 14 strikers.