Dicers trying to say this isn't a huge blow are in denial. There may be customers, but this rule has major implications:
1) MUCH harder to advertise / promote. Say goodbye to making Youtube vids with your RSN showing your big pots (unless you don't mind a ban)
2) No advertising in-game. Sure, many of the hosts have an easy time switching over to IRC. However, the profit is not made from hosts, it's made from regular players. It will be much harder to find these regular players without advertising in-game.
3) Trading GP is now much harder for everyone. Without gambling, it will be easier for Jagex to detect RWT trades. When you're IRC dicing, you can be banned now for RWT with the unbalanced trades.
All together, this is a MAJOR blow to gamblers, and a minor blow to RWT as a whole, simply because gambling trades made it harder for Jagex to detect RWT.