I think it's both.
You really can't blame the "Mental Boom" by itself, because the team is legitimately inting/feeding/afking/already ignoring every objective in the game. If they aren't at 4-8 minutes when the slowest yet most important decisions are made, then they aren't going to pay attention when the game is moving 3 times as fast either.
Likewise, "Mental Boom" does have its place as a culprit. I'm VERY "Mental Boom," because I'm probably Ausbergers, and my brain processes and predicts everything around me better than my peers. I see every death before it happens...... 30 seconds before it happens. I'm calculating the gold advantages a feeder is giving a hyper carry. How many minutes those deaths shave off the time to get a Shojin's Spear, or a Sterak's. How to still gain gold when a support is outright csing.
So yeah. I do point out mistakes that are serious. But it's near impossible to teach someone how to play on the fly if they don't already know (A 0/6 Nami who is full building her support item, takes 16 minutes to complete Quest, and then starts building full mage probably doesn't know how to play, Especially if she face tanks a Warwick), and any advice you DO give is taken as toxicity. Everyone you are talking to is purely innocent, never fed, and somehow it's your fault they fed.
But yeah. I have seen the results of my own "Mental Boom."
Result:
1 in 10 teams will outright troll you, figuring they were going to lose anyways.
2 in 10 teams will passively farm the game away, opting not to troll entirely.
4 in 10 teams will actually take it constructive, but report you anyways.
3 in 10 teams are indifferent to it.
Imo, it leads to more wins, but also more reports. Eventually the wins will stop because of the reports, chat bans, and later bans.