I think a common ground between Gilmunds idea and dmdelors concerns is a good place to start. It's weird a 1 man unit still has the strength of a 10 man unit, shouldn't the strength be based on the losses taken? We lost our 2 mg 34 gunners, no more MG, we lost our 2 MP44/Thompson gunners, no more thompsons, etc...
I do think reinforcing was taken out for a legit reason. I did love taking a town or forest with elite units, filling those areas with line infantry to hold the next attack and sending those elite units to rest and refit. Let them sit for a while and reinforce via the helicopter, it always took a pretty good amount of time to do that. Then bring another HELO over and get them back in the battle! Was a great feature!!
Anyway, maybe no veterancy gain in battle but maybe refilling units within X amount of distance from the front line, and having their veterancy suffer a bit based on the amount of new recruits. Vet 3 unit with 2 guys left becomes a vet 1 1/2 unit after 8 more fresh recruits come. The formulas would be ez.