Anecdata, but I have gone from 100% checkin to almost 100% carryon because I'm simply not interested in paying one more fee. I'd rather have someone handle my bag for me, especially if the alternative is schlepping it through multiple connections, but the airlines have incentivized people to carryon instead. It absolutely slows down boarding and fills overheads. This is probably why Southwest still allowed 'free' bags; they turn around planes quicker than anyone else.