I think more will play if quickly trying out games was easy. E.g. I wanted to try out Doom Eternal yesterday without having to buy it first in case I didn't like it; couldn't find how to do that and gave up after 20 mins.
Back in the day when shareware was a thing it was much more prevalent with game demos, but that was also before game consoles really took over. Then physical stores made a difference, where you could go and try out a game before purchase, but of course those aren't really much of a thing anymore.
My conclusion of the research then was that most games are simply not built with a demo in mind. It's more common these days for games to front load the introduction part of the game, often a tutorial, so that you can start playing before the game is fully downloaded, but back then it wasn't very common. This could work as a stand-alone demo and I know a few games that does this – Tomb Raider for example – from which you could then unlock the full game and download the rest. But it was (and might still be) an exception to the norm.
My opinion is that online stores can and should put pressure on game devs (and engine makers in particular) to enable customers to play a demo of a game before committing to a purchase. It might not work for all kinds of games (particularly ones without single player elements) but I reckon it ought to work for a majority of them.
I know it's not a good old demo.