By the time COH3 will be out majority of the now dwindle RTS crowd will be in their late 30s and 40s. The new kids love that Fortnite BS these days.
Unfortunately, you're probably right (partially; the RTS genre is likely too niche to for CoH-type "$60" games to be profitable (but I'm no market analyst). While I do think there's some sort of new CoH game coming, I don't think it's going to be the "CoH3" many people are expecting.
I don't think it's an issue of "new kids" not being interested in RTS, though. It's likely just a naturally declining genre; "Arena Shooters" (Quake type games, etc.) have had similar issues in recent years, despite there 'supposedly' being a large demand for modern games in that style, and those are very fast pace 'twitch' shooters that many would expect to be popular.
If there ever is a CoH sequel, I'd expect it to be an F2P "CoH2 relaunch". This would mean recycling a lot of the underlying assets (art, UI, code, etc.) to keep costs down, but then adding a bunch of new features/modes/"Cosmetic MTX" to drive interest and profit. Basically, I'm expecting another attempt at 'Company of Heroes: Online'.