Your anecdotes might be true, but your conclusion does not align with empirical reality. I've seen multiple separate German studies looking into renter satisfaction towards their apartment/landlord, and large companies consistently rank lowest, while private landlords are consistently rated much better (sometimes first place, sometimes after unions and municipal offerings).
It's great that your last landlord company treated you well! But that's not the experience most people in Germany have. For example, I personally know >10 individuals who rented from Vonovia and deeply regretted it, because there were so many issues that maintenance only covered up superficially while NEVER fixing the actual issue. Even for severe issues, they usually waited 3+ months before sending anyone out - in one case a wall showed obvious water damage & mold, and it took 4 months PLUS a letter from a lawyer before anyone followed up on their dozen reports!
> Economies of scale is what allows these big companies to offer better prices and better service than private landlords.
Sure, that's the theory. But when reality does not align with theory, something must be missing: your model only works as long as the market fulfills certain conditions, which the housing market in Germany (and likely most of the west) does not fulfill. If you haven't heard of these assumptions, I'd recommend looking into non-fungible & inelastic markets.
Because previous attempts at discussing such topics were usually met with hostility, I'll only go into more detail if you can bring empirical counter-evidence, or if you were sincerely unaware of the limits I've mentioned.