Honestly, I see way more toxic behavior in regular/heroic dungeons, LFR, and BGs than I do in M+. Sometimes Consumption in Korthia ALONE is more toxic than my entire week of runs.
This has generally been the case with high-level raiding too. Not sure what your point is. The highest level group content is done by players that play together frequently and communicate. Not sure how that's a bad thing?
You can totally progress through M+ up to maxing out your vault reward (+15) or higher without any team or scheduling commitments. Sure, once you're in the +20's and well beyond the max gear rewards, it's harder to progress- especially through PuGs- but you can absolutely "drop in any time and...progress." It's literally what I do on most of my toons.