We do have counter play for Demos, minesweepers. This might sounds odd, Whermacht/OKW has to adjust their strategy with infantry pushes supported via mine sweepers. CAS has no counter play what so ever, but that is another topic.
Demos require 1 CE squad out of combat for about 15 seconds.
The counter to demos require a pioneer/sturmpio with a mine sweeper with EVERY squad (and tank) in combat for the rest of the game because they could be anywhere on the map.
Yes demos have a counter. Do they have a feasible counter? No.
This may be hard for some of you to understand, but demos actually (imho) encourage blobbing because you can protect your whole army from demos with 1 minesweeper. If you didnt blob you would need to buy a sweeper for every unit that moves on its own.