Okay, I'm aware that my opinion matters as little as anyone else's, but I'm gonna throw my two cents into the well.
Browsing the Antstream games list tells me instinctively that this is both by and for British gamers: hundreds of games that most people outside Europe have never even heard of - and on systems that many Americans haven't heard of either. Believe it or not, this immediately adds to the pull of the service: the big consoles from Japan are well catered for in terms of official playing options - either on consoles like the Switch or on mini consoles from Nintendo, Sega and Sony. While there have been attempts to do the same for 80s computers, it's nowhere near the level that Antstream is attempting.
Sure, we can all buy a 6TB hard disk and get every game ever made, and fine-tune various emulators to play them, but the success of mini consoles and the like should tell you that the average consumer can't be bothered with all that. When they say "fiddly emulators", they're right: a 40-year-old who remembers playing Flimbo's Quest isn't likely to go Googling for a D64 file of the game (trying to figure out which of the six different files is the best one to use) then go download and unpack WinVice or whatever equivalent for their platform of choice, then set it up. The prospect of "just instant streaming" on multiple platforms is actually an enticing prospect for many people. Not for me, but then again, it's not aimed at me.
Regarding preservation, and how terrible streaming subscription services are and yada yada, forget it. Every game in that list is well-preserved, so that's not an issue, and if you don't like streaming subscription services, fine, neither do I, so I won't be using the service. That doesn't mean it won't be interesting to a lot of other people. No need to leave downvotes on a YouTube video - pretty childish, if you ask me.
Finally, as I said before, you CAN download every game already - we all know this. But some people want to go the legal route, and fair play to them. In that case, the fact that they're making an effort to acquire the rights is significant: without this service, there is literally no way to legally play a large amount of these games. Again, we can all go and stick them on our HDD and not care less, but I think it's fair if there's at least SOME consideration to doing things legally.
The one thing I will say is that it appears the service will stream video to your device in order to play anything - so it'll be like Google Stadia, except for games that are under 100KB. Hmm, yeah, that doesn't make a whole lot of sense, from an efficiency viewpoint. They could just let you download the file locally and use no bandwidth, or potentially have to send you gigabytes of video. Not to mention the input lag, which I think will always be an issue for this kind of thing. Still, again, the super-casual people will be like "wow, Impossamole on my smartphone" and not really care about those details.
All in all, I think the people who have the most vitriol need to calm down a tad. If it fails, it fails: no need to stomp on its grave. But if people like it, let them. Don't worry, nobody's coming to take away your 6TB hard disk.