They could easily do so on a console or game streaming service, just give you like 2 hours and then switch it off.

I think Sony actually do that as part of one of the PSN tiers.

But I think the main driver behind no longer doing demos is that when they started analysing it, they found it mostly reduced sales. A lot of people were no longer interested enough to buy it after playing, at least not at full price. I gotta admit, back when demos were common on the front of magazines, there were very few that I actually purchased on the basis of the demo. The ones I did buy, I’d have probably got anyway, like Metal Gear Solid 2.