I've played and enjoyed action games made with BYOND that use framerates above 10 fps, so what you're saying can't be entirely correct. That's what makes this a gray area - there are some multiplayer action games that would work on BYOND and some that wouldn't.
I'm not even talking about entire games here, even something as simple as two mobs moving around a flat area with pixel movement and nothing else. You could have high speed internet and it would still be choppy and unresponsive at times. It might work at 2am when everyone in your neighborhood is offline, but most of the time, and for most of your players, the latency issues will ruin the game.
Building something awesome only to have it fall apart because of something you can't control is much more devastating than you might think. Many promising projects have most definitely been abandoned because of these problems.
> I usually see at least 6000+ people on at any given time.
http://imageshack.us/photo/my-images/507/screenshotod.png/
Yeah sure, over 6000 people at 5:14 P.M (UK)...