I think with better accessibility of video reviews and let’s plays of games, where you can see actual gameplay, the need for demos decreased. At least I do not really miss them that much. I also think the size of today’s games contributes to it. If I have to download 80 gb to test a game for 20 mins to find out I do not like, then I prefer to watch a quick video to get a first impression.
On the other hand, I played the ut2003 demo for months back in the day. Did not care that only a few maps were included, and my pc was too shitty for the blocked higher graphic settings anyway. That was a great demo.