Going back to OP's proposal.....
I would think it's very much 'realistic'....but than again we get into the old argument about realism vs. gameplay. That much of restriction would definitely make things much more tedious and suck a lot of fun out of ETW.
Further more......let's not forget the crippled AI. They can't even conduct naval invasions now..if this restriction is set in place they wouldn't know what to do (and even if it was patched up, they would be sending piecemeal forces). Nah....Just leave it as it is. It evens out IMO with the fact that all your guys and stuff are lost if you happen to lose that single brig.