I personally could care less if it was never deployed as long as they had a prototype. My concern is the same as yours, once you let one random make believe unit in you let them all in and it is an easy way for relic to implement pay to win.
It does not necessarily have to be "pay to win", but it would just be vehicles that really only have been prototyped and then scrapped. For the Black Prince that was based on a heavily used tank, we can at least estimate how the performance maybe >could have been<. Still, it would be mostly made up, even if you have some performance data sheets. And to take the fully over the top example of the Maus tank prototype:
This thread is weird though as people want historically accurate but this game is not accurate in the slightest. Panthers should not be diving with their weak side armor, KV series should not be competing with late war tanks. Hell, right now the P4J is probably one of the best meds competing with the T3485 and E8. When it was first introduced it had horrible turret rotation to simulate german late war lack of resources. It was buffed since it sucked because balance. Similar situation with ISU152/IS2 stun mechanic, the gun wouldn't penetrate as often but the stun mechanic kept them in the fight allowing for support to come in and do their job. This game doesn't even attempt to simulate the upkeep of fielding heavy tanks, the closest was the Tiger ACE.
What people see as "acceptable" will depend on how much they are into history and therefore obviously cover a very wide range. As you said though, CoH was never accurate or realistic. Instead, it was more "authentic". At least most weapons follow somehow the behaviour you would expect, given some gamification and balance circumstances. Everyone has their personal shtick though, and that's what they will pay attention too. I find it tedious to argue about complete historical accuracy though. The tank/gun/whatever does not need to be modeled perfectly, even less it SHOULD be.
In the end what most people can agree on is that the CoH series claims to tell the story of historic events. And as such, it should stick to its time frame. Implementing units that have actually been used in WW2 and had at least some tactical significance is a decent guideline I guess.
Of course there can and should be some more crazy and fun stuff, but there is already enough to work with within the WW2 time frame.
Regarding the Black Prince, I find it baffling why Relic actually tries to push for it. Yes, it is somehow unique, but in the end investing resources into creating the Black Prince means not creating another model that could have been accurate and opened up its own strategies.
I can live with the Black Prince in the game. I don't have enough of a shtick to rage about it. And since it looks like a Churchill, I can mentally easily label it as one. But this doesn't mean it should be there, if only for the reason to prevent Relic from introducing stuff that stretches the "WW2" theme even further.