That invalidates medium armor though.
Not any more than it already does... In RAW:
Light armor and DEX +5 caps out at 17.
Medium armor and DEX +2 is also 17.
Heavy armor and DEX +0 is at 18.
So, if you go with a high DEX character, light armor is as good as medium.
Just so you have the entire picture:
Light armor uses full DEX, so caps out at 17 still.
Medium armor uses half DEX (round up), so now caps out at 18.
Heavy armor uses half DEX (round down), so now caps out at 20.
MAM feat allows you to use your normal DEX mod, but to +3 as usual. In other words, it isn't halved.
Sure, heavy armor can be better than medium (oh NO!, say it isn't so!), but requires a heavy (no pun intended) DEX investment
and STR investment to get there. You need at least a DEX 14 with our house-rule to even get a +1 to AC in heavy armor!
So, no, it doesn't invalidate medium armor. In fact, we only have one character in heavy armor in our group, the rest have medium lol.

(well, out of those who
wear armor I should say...)