There's massive cost differences between streaming a video and rendering games and streaming them.
Perhaps they plan on making up that difference simply because they will make ~$20 or so off of a game sale, but then again people also play games for WAY LONGER than they play a video they buy.
So it's quite possible they'll make you watch advertisements; if not to make up costs but to provide a greater value to the paid sub.
I'm only saying it's a possibility; unless they specifically said it wouldn't be ad supported you can't expect it not to be. I imagine the "comes next year" aspect might literally be to make decisions like that, partly based on the actual costs associated with the users of the paid subs.
You seem to have missed the part where the "paid sub" isn't even a "humble bundle" at launch but "one free game" and higher resolution / surround sound. They aren't just charging you because of game licenses, they are charging you because of the massive costs of game streaming relative to other web based services.
Its Destiny 2 + what looks like all the expansion packs (including the unreleased one) as the 'free' game at launch, but it also says its going to be regularly adding older games to the service, exactly like PSN / XBL / Humble monthly do. And I don't think its a stretch to assume a chunk of those 'free' games will be indie titles that don't need a GPU, so can be run on pretty much any VM, not one of their gaming specific bad boys.
The $10 monthly is primarily for people who want 4K HDR streams, AFAICS, and people who want 4K streams are already paying a premium for that on regular video steaming services, so I don't think thats exactly a value deal breaker either.
I don't even think they need to 'ad support' F2P titles; most F2P games have moved to a high engagement, regular low cost purchase model, which they'd get a presumably 30% cut on for every lootcrate / skin / exp booster purchased. Its not like 10 years ago when it was 'big payment' purchases targetting the 1-2% 'whale' userbase.
e: I assume its Destiny 2 as their 'big name' because Destiny 2 on Pc is a huge fucking flop and
so they got a great deal to try and push it