Now I understand you are sad not every class, subclass, race, specialized build including also all kind of Wizard builds and Cleric domains of every class from PHB 5th edition will be availabe at release.
Actualy i was looking forward for your (and anyone else really) explanation for at least one reason ...
That is why i asked: "Why it would be not realistic?" ... yet you decide to ignore that question and instead just stated that your opinion is a fact, more solid then word from the actual leader of actual development. O_o
I am sorry you have to adapt to that fact.
No i am sorry, but you will have to adapt to that fact, that just bcs you believe something, it dont makes it fact.

The thing is, ultimately this argument is Larian claimed they'll do it, and you are claiming they will not.
Please, bring the proof of Larian claiming that.
Will this link be sufficient?
https://www.reddit.com/r/baldursgate/comments/fhq9cq/compiled_baldurs_gate_ama_20200313/I shall quote few interesting questions and answers, but feel free to read it whole ... its quite interesting imho.

There have been hints of other 5e sources being implemented into the game. Are you going to include material from Mordenkainen's Tome of Foes, Xanathar's Guide to Everything, and/or other player materials outside of the core material of the 5e Player's Handbook? Additionally, at full release, will all content of the PHB be represented (notably, (sub)classes and (sub)races)?
Swen: HB/MM/DMG are the base we are starting from but we are taking material from other books too.
At full release, will all of the PHB classes and races be represented?
Adam: At launch we’re planning to include all classes from Player’s Handbook. We’re only talking about the races announced during the gameplay presentation for now.
Will all 5e Classes be available at launch?
NickP: Yes, all classes from 5e Player’s Handbook will be included at launch. In Early Access you’ll be able to play as a Cleric, Fighter, Ranger, Rogue, Warlock or Wizard.
Does that include paths, subclasses, etc?
NickP: Yep.