We toyed with this idea a while ago, I can't find the threads now but I sure they can be searched for somehow, but in the end we decided not to impose such limitations.
Forced upgrades would damage a collector's ability to... collect favorite units, just to keep them as trophies.
Making units obsolete just because they are no longer in production is a slippery slope. For example, production on the Tiger I was halted towards the end of 1944 as all remaining resources were poured into Tiger II production. Does that mean we remove the Tiger I from buy menu and force players to remove Tiger Is from their core? That's harsh to say the least.
Many players enjoy using NOT all top of the line equipment, it adds to their sense of historical accuracy and can be used as an interesting way of increasing the difficulty of play.
We did add expiration dates to some units though, as players have mentioned above. We also added them to several SE units (because SE units are awarded randomly from ALL available SE units) but this was to prevent getting a... SE Panzer IIIF as a reward for starting Bagration.
