I can't satisfy your curiosity because I too also sincerely don't know what flavor of undesirable behavior a monopoly creates but that they create bad behavior is as intuitive to me as is breathing to anyone else.
A first person anecodote which doesn't necessarily further my point but still - I had Jio fiber installed for a relative, I was visiting in a small north Indian town - unbelievable price, offering, everything great. But it took me 3 days of multiple hours on the phone, and I still couldn't access Hackernews, or reddit or even New York Times. Pages for these and quite a few other sites just wouldn't load. The first canned response - 'Reliance only blocks what the Govt. has told it to block' (paraphrasing). I didn't anymore have the energy to analyze if it was a technical problem, casual censorship or just customer service issues because of fast scaling.