I was going slightly off topic - sorry for that -, but I think I at least properly responded to one of your posts. I will keep it shorter and more focused in this one.
It does not really matter how many different units are available if "last tier rush" is the optimum strategy. Unit/tech that delay that strategy will simply be avoided.
Apart from maybe the very first versions of the game (I did not play on release, but what I vaguely remember from 2014/15 is that Soviets had a split tech path and the T34 for example was in T3 or so), the game has been designed for a last tier rush ever since. The only exception being Ostheer since Osts T3 can actually deal with most of Allied armor.
That is not the "base assumption" nor is this a thread claims that everything was roses when Relic was doing patches and everything is bad when the MOD team took over.
The basis of this thread is the patch notes (included in op) and series of changes over in many patches that are have reduced tech cost and "back tech" cost (I would add here making many doctrinal vehicles available only via last tier). This clearly makes a "last tier rush" strategy more efficient.
You said since the balance team took over tech paths have become more linear and therefore builds less diverse, which implies that it was the opposite before (I never said you were assuming everything was perfect back in the day, but essentially this thread exists because you think CoH2 got worse at least in this special regard).
I agree: Units have become more available in general. Mostly though, changes aimed to give every faction similar tools (all factions should have access to MG, ATG, mortars, a source of smoke etc...). Back teching often does not diminish the window of opportunity for units. If, e.g., A Soviet T1 player backtechs to ATG in the mid game, neither the windows of opportunity of Luchs/Puma nor of a P4 are diminished.
You cite CoH1 as a good example of needing to chose techs exclusivle (I never played it so I can't judge). But for the most part these exclusive tech structures just brought a lot of problems for balance and are partially still not solved (SOV, USF and OKW all T1/2).
Even Sander93 posted did not deny such a trend and said: "incentive has always been there"
Relic was never a fun a the "smallest" steps, on the contrary Relic was notorious for overbuffing and overnerfing things in their patches.
Neither did the MOD patches used small step (at least early pachtes)on the contrary some of the MOD patches where the most radical ones. [will snip the rest for brevity]
Sanders post makes it unclear whether he is talking about balance team or CoH2 in general. Because in all versions of CoH2 the lighter vehicles got more and more useless when heavier versions came out. The Scout car was never useful past minute 6-7+, the 222/251 were also only useful until another "heavier" LV hit the field and even more diminished in usefulness once medium tanks arrived. There have been some other strats (like SU76 spam) that worked as well, but many were based on call-in strats that bring their own problems as well.
Regarding the "smallest steps": Balance team changed tech paths as much as needed (see USF), but if there were two potential solutions to a problem, they usually took the one that costs less effort, would be easier to fix/revert if it turned out bad or the one that had the most predictable consequences (in the end, these were mostly stat tweaks).
To repeat myself: In this game, there has always been an incentive to tech up and get better armor on the field. The only way to not play like this are rush tactics were you put all eggs into one basket and can insta-forfeit if you fail the strat.
As the last point, I want to come to what - I think - is the core of this discussion:
Lighter vehicles become less useful the longer the game goes, sometimes even to the point where it is almost better to throw them away. If light vehicles were useful in the late game, there would be less incentive to get your highest tier up asap. But they are not, so every one wants heavier tanks quicker than their opponent.
In other threads you often suggested to prolong game phases by delaying units. I already remarked how this would cause reworks for the power level of most units although I generally agree. But in the end it would not solve the problem: Last tier would still be top choice, you'd just need longer to get it. And the player that can use these units first wins if the other one does not tech up.
The only real solution is to increase late game utility of early units. I once made a whole thread about this so I will not repeat myself now, but balance team also implemented a ton of these utility buffs for light vehicles and is still expanding them in the current patch.