Demos are a bit stupid imo. Everyone should have access to them or no one should have them.
Same could be said about heavy armor, huge range, indestructible tank destroyers, variety of potent upgrades, handheld AT that actually destroys tanks, 5 vet levels and so on.
At least make them targettable and destroyable by small arms once a minesweeper finds them.
You can disarm the demo if opponent can not see it. Once disarming starts he can't detonate it. Just disarm it when you are 100% certain he can't see you and do not know you are there.
Obviously demos wouldn't be needed if allies had infantry that actually scaled in the late game and could take on in direct fight axis infantry and since that won't happen, I wouldn't expect demo changes.