I didn't say we would never support it. IMO It's a trickier subject than most people are giving it credit for.
Issues with dt at low framerates are a different problem. If the framerate is low enough to cause problems with gameplay, usually it is much lower than 30 FPS, so a framerate cap does not fix that. It's a separate problem.
Adjusting the framerate cap at runtime is a pretty difficult problem and hard to get reliably right. You'd want to figure that out on startup, but Javascript optimisation and garbage collection tends to make things a bit choppy on startup, so any measurements would be unreliable. Even so later on there might just be a single momentary blip below 60 FPS, and you probably don't want that to trip it down to 30 FPS. So then you have to draw a line somewhere, and wherever you draw the line, sometimes you'll choose 60 FPS when it should have been 30 and 30 FPS when it would have been better to choose 60. I'm not at all confident the engine would be able to choose correctly most of the time.
Other game creation tools might provide a framerate setting, but I don't think that makes it a good idea!
A screen with a refresh rate like 72 Hz doesn't play the game at 60 FPS - it plays it at 72 FPS (if the system is fast enough). A framerate independent game adapts to this seamlessly, ensuring best quality display. To be more correct, instead of talking about 60 FPS and 30 FPS we should say "vsync rate" and "half vsync rate". The engine could support a "half vsync rate" mode, but then you can't easily tell the display refresh rate in Javascript, so you have to go off some adaptive system again, which is unreliable and might pick wrong, and then you have a choppy framerate again. Also that kind of thing needs reliable high-precision timers, which aren't available everywhere.
Consoles are a different case: the hardware is known in detail in advance, so you may be able to say confidently that a game you have absolutely cannot run at vsync rate. However C2 games run everywhere from powerful desktops to weak phones. IMO a game running at half-vsync on a powerful desktop PC is both annoying and an inferior experience to a vsync-rate game. I didn't spend all that money on a ridiculous graphics card for 30 FPS :)
In short my opinion is that if a system can run a game at vsync rate, we should let it. Since there's probably not a really reliable way to detect the need for half-vsync rate and then accurately hit that, the best way to try and let it is to leave the framerate uncapped.