Sevatar, your whole idea is based around the premise that the current game doesn't take the evolution of tanks and the technology gap into account.
But in fact it does by giving older models a higher availability.
But that's all sorts of screwy. In 1991 you're going to have a harder time finding an M60/M1 than an M1A1+ in the US inventory. The legions of T-34s for the not-DPRK users are only possible if you took them off of display stands and out of museums. Indeed, by the time of mid to late CAT-B to CAT-A Leo 2A1s and M1IPs are going back to the factory and coming back as Leo 2A4s and M1A1/M1A1HAs.
Older doesn't mean there's more available, or else we'd be able to get 1.5 million Shermans in a deck. The evolution of tanks involves the obsolesces, extinction/retirement of earlier tank designs, and new designs becoming standard. This is distinctly at odds with making all units worthwhile, but a gamemode that uses "authentic" decks would work well in terms of giving people who want clashes of titans, the late Cold War reality of M1A1HAs, T80Us, and Leo 2A4s in some sort of gotterdammerung fight to the finish, while not forcing people who are in love with the T-72M to face down dozens of late model tanks. Unless they like getting annihilated.
We'd have to be careful when excluding CAT-C/B from later tier games. Some countries held onto their older stuff for fairly long (like both the Chaparral and PIVADs both lingered well into the mid 90's despite being totally outdated). Having an "exit" date independent of the availability date is pretty much essential.