I see streamers opting in these kinds of streaming aggregators in the future. With the advent of all the new services later this year and beyond, streamers will have to return to the bundling method and will take a price cut to ensure they are included. Maybe HBO still charges a premium, but if you look at the list of channels, many of them can be bundled. See Acorn, PBS, Tastemade, MTV, DC Universe, Sundance, Epix, Smithsonian, Lifetime Movie Club, UMC etc., etc.,
I agree that there will continue to be some "long tail" niche services that aren't part of a huge media group and therefore can't or won't run their own apps and instead must rely on aggregation platforms such as Apple TV Channels and Amazon Prime Video Channels for distribution.
But some of those niche services that you mention will end up belonging to a major player and getting subsumed into their main direct-to-consumer service with its own app. For instance, the upcoming HBO Max service, which is going to be WarnerMedia's BIG service that goes head-to-head against Netflix and Hulu, will contain all those new originals from DC Universe (which itself may just shut down). Viacom's mini-services like NickHits and MTV Hits will probably just get sucked into CBS All Access after CBS acquires Viacom. Sundance is part of AMC Networks (along with BBCAmerica and IFC) and that little company is RIPE for acquisition by someone. (CBS? NBCUniversal?)