It's a little difficult to reasonably base fan support by comparing attendance from 20-30 years ago to today. The in-home viewing experience was nowhere near what it is now and access was severely limited by comparison. Also, the target audience is made up of different people with different socio-economic demographics.
To illustrate the former, I remember when I was a kid, going to a game was an event. We didn't have NESN when my family first got cable (Greater Media Cable didn't even offer it on basic cable until the early 90's IIRC.), so watching a Red Sox home game likely meant you were there. When watching a game, it was on a 19 inch tube (at best) on a UHF station and lacking vertical hold...and an away game.
Nowadays, access and quality of the in-home experience allows people the ability to prioritize any number of things over in-arena attendance. I don't think it makes them any less fanatical per se, but personally, if given the "choice" between an event my kids are involved in or going to the UConn game...Don't get me wrong, I'd still rather go to the game, but knowing I can DVR it, go social media silent, watch it later, and not risk being served papers during the after 12:00 media time out, you better believe I'm going to soccer practice.