Ideally higher tier kit is perhaps stronger overall, but the older kit is still preferable in at least some situations, so keeping it around or pulling it out every once and awhile makes sense, even if you replace most of it.
I think the first three axis tanks in normandy actually illustrate this fairly well.
The first thing you get is the Puma, which offers excellent mobility and reasonable anti infantry firepower, but lacks in protection or antitank capabilities.
The next one is a panzer with better protection and excellent HE, but only marginally better armor penetration, and a massive sacrifice in mobility.
Then you get tank number 3, which has even better armor, and finally a gun that can really punch through allied tanks in a way the others struggle to. However, it can’t clear rooms with HE splash the way the last one can, and it can be difficult to get into position or avoid being flanked by infantry.
So, with those vehicles, if you have all three available in a match somehow, any one of them will likely end up being the “optimal” pick at least some of the time, even if there is still a clear progression of “Bigger and badder” as you go along the tree.
Another example IMO is the winchester vs the SVT in moscow, at least when neither rifle is upgraded. The SVT has a much better fire rate and magazine size, of course, but it actually has accuracy problems even at mid range, and often downs instead of killing. The winchester on the other hand has excellent accuracy and high per shot damage, with a fire rate that is “good enough” for mid range combat most of the time. So, while the SVT can be more powerful, and usually is, there are times where the winchester is still a better choice.