If it's an exodus like this, something tells me it isn't the game that's the problem but the company. Then whether it's company structure, the company's relationship with Microsoft, even work culture, I don't know.
Speaking of ETS2 though... I'd argue the game's open betas are proof that PG could release cars after their dedicated series without losing "player engagement". Why? Simple - early access is a surprisingly powerful thing. ETS2 open betas attract a bunch of players, even ones who aren't very interested in bug reporting simply cuz they get to enjoy some of the perks of the upcoming update (map reworks, new gameplay features etc) early. If PG added playlist cars in a latter series through say wheelspins (if not the series directly after, maybe the one after that?), I would not be surprised if a similar phenomenon occurs, with player engagement for the series being impacted very little (if at all) and with players being able to access cars that they couldn't get their hands on in time maybe it'd quell atleast some of the negativity around the game. Sounds like a win-win situation in my book.