It’s a market for lemons [1]. You, the player, have no information about the teammates and opponents you’ll be matched up with. Therefore, over time, high quality and well-behaved players tend to exit the market, leaving only the cheaters and tantrum-throwers.

[1] https://en.wikipedia.org/wiki/The_Market_for_Lemons

When someone tells me something that's been happening for a long time is suddenly going to stop happening, it's most believable when they present evidence that is the case.

Being that humans have been cheaters in one way or another in significant numbers. For example modern infidelity figures are around 25% based on surveys. Somehow I just don't think your statement here is backed up by observation.