I would kill for either a Medieval 3 or a reworked Empire…and CA have to know that.
For the past 3-4 years, every single player poll I've seen -- whether it was in the forums, on Reddit, or elsewhere -- has resulted in Medieval 3 and Empire 2 being the two top choices by a country mile, with M3 usually just edging out E2. If one of those two games isn't the next major historical title, CA is almost certainly going to have a lot of (even more) pissed-off customers.
Is it verified/bonafide that WH3’s release was indeed a true trainwreck?
Depends on your definition. In terms of sales, I don't think there's much question the game did well. While CA/Sega don't typically release numbers, Warhammer 3 was Steam's top-selling game for something like 3-4(?) consecutive weeks when it launched back in February, plus it had a large number of pre-orders, though we don't know how many. Even if it WH3 hasn't sold as well as its predecessors, it's still made CA/Sega a lot of money.
However, if one defines "trainwreck" in terms of quality of release, there's likewise not much question the game was released in poor shape. There have been tons of issues, ranging from technical problems, to gameplay issues, to even core elements and/or fundamental parts of the design being poorly thought-out (which at this point can't realistically be fixed). And we won't even get started on the Immortal Empires campaign -- the part of the game the vast majority of players wanted most -- not being available at launch (and almost certainly won't be ready for "primetime" until next year)...
Suffice it to say, the DLC team has a ton of work ahead of them. I do not envy their jobs.