In general, at all the analytics data that I have seen for my assets piracy does not really hurt sales. Even if it did, there is not much you can do about it when you’re content isn’t DRMed (and even then if it becomes popular someone will crack it). As far as I’ve seen in our field even the large corporations can’t effectively stop the torrents/websites and it is a never ending game of whack-a-mole. Epic and Unity certainly do get take-downs for the popular pirate sites even if they aren’t hosted in countries that a small company could realistically do on their own, but it is usually just a hiccup and everything (meaning the whole library of pretty much everything on either store) is re-uploaded somewhere else within a few weeks. These are seeded from only one legitimate purchase. Having a WebGL demo won’t make it more likely that your stuff is pirated, if it’s even remotely popular you can almost guarantee it’s on a pirate site somewhere.
In my particular case even from the larger groups like cgpersia it really only accounts for about .005% of traffic. Adding all of the known pirate sites together still doesn’t reach half a percent. On top of that, if you just follow the normal logic that most pirates were never going to buy your stuff anyways so it really doesn’t effect anything. There are even Unity publishers that use it as an alternative marketing means seeding their own content themselves (usually the developers that add stuff all the time so that the torrent is usually a few versions behind so even other seeders can’t keep up). Granted, this isn’t really a challenge currently for Unreal since the market isn’t that large so using it as a means of advertising isn’t really necessary yet.
If one is really paranoid about that stuff though, your best bet is that approach of the ever evolving product (if its art add to your art packs, if its code keep adding features). Not only do you build a really loyal customer base, but you make it hard for those that would pirate who will always be behind what you’ve added.
I don’t think that webgl demos should be required but if Epic’s marketplace ever takes off then when saturation comes into play most publishers are going to want to add them anyways to stand out and increase sales.