Compare to when? Release+1-2 years? Maybe. But you yourself said that these versions are so old you don't want to discuss them. There were true medium rush metas 2-3 years ago. Also as Aerohank said, LVs have become mandatory or at least meta 2 years ago so there seems to be incentive not to rush mediums/final tier.
Stalling for call in has been a viable strategy until it was completely removed around a year ago.
As already explained that does not mean that tone will go for their final tier without building any vehicle at all. One will simply the way that allow him to get the there the fastest.
Multiple people have provided their opinions about your claim as well as the alternatives, yet you brush those off as "not the point of this thread".
They have not provided their opinion on weather tech should be a simple time limitation and "rushing for final tier" should be encouraged or not.
Only Aerohank and mrgame2 have provided their opinion. Even you have not provided your opinion on this question. Do you agree with Aerohank who thinks making all faction having a similar tech system in game would fine:
"I wouldn't mind if in CoH3 each faction had an Ostheer-like tech design. With Ostheer you have to make strategic choices on how you spend your resources, but you have the freedom to assign these resources wherever you see fit. "?
You also made a lot of claims and do not back them up.
How does making mostly backteching more available decrease unit/build diversity?
It does not. What it does it making rushing for final tier more viable since one can gain the bonus of the final tier like: MR, all out war, Anvil/Hammer bonuses, Major bonuses with sacrificing less.
It becomes more of "braindead" decision instead of calculated the pro and cons of the options.
What reduced the unit diversity is overlap of unit from higher tier. One will simply not build a SU-76 if the SU-85 is available and it clearly a better choice.
What reduces build order diversity is if there is gap between tech choices. If the a single tech is so impactful that it is must one will simply choose the build order that allow him to go for that tech faster.
There are some points to be made for this but a lot against, and you keep ignoring the ones against. What are the alternatives? You don't discuss any of them although others and I have pointed towards them. And there is absolutely no use to saying a direction were "good" or "bad" if you neglect the alternatives.
There many alternatives:
Keep MR in T3 now that it buffed so the player has choice when and if to unlock it.
Keep AT grenades and Molotvs separate so a player can choose when or if he want to unlock them
Make certain unit call in (not mainline tanks) so player have the option to use them instead of
having to use their final tier.
Make end tier unit more specialized that they do not make lower tier unit complete obsolete.
In the end of the day make a decision and stick with it, instead of adding bandaid on bandaid:
Either make all factions with completely linear tech like Ostheer and designed them accordingly
or
Accept that some faction will have holes in their roaster and make sure they are viable even with those holes...