If OSRS gold does go on Steam, they will surely get enough new players to make enough cash that the 30% are a non-factor. Can't talk for Epic though even though it is only 12%, they might not be able to recover the cost if they go on there. It might be moot. Game stores give access that is large but do not have player retention that's required for an MMO.
It could be quick short-term profit, however any new person wouldn't be anticipating this kind of game to maintain the shop leading to only F2P or just 1/2 month subscription. You then would need to deal with the negative reviews that are likely to come out of them because RS is a very slow build up to end-game content compared to the typical action-packed or speedy gratification of the other matches on the platform. Would being on steam give enough money to be worth it? We are talking about the cut, we're talking about the money spent making RuneScape fulfill the requirements steam has.
I really don't understand exactly what they are but they're at least likely to include writing a launcher that may work through steam. Steam distributes updates for games on the stage so you have to have a way to get the upgrade through 2 channels at precisely the same moment. Not impossible tasks although not 5 minute types either. And that means you've got to ask, what would it do? Runescape's problem is not that people haven't heard of RuneScape - it's that people don't want to play with it. Even people who have never played an MMORPG in their lifetime have heard of runescape.
The problem is getting them to play, would being on matters like steam assistance that? Perhaps, but it is not certain. Steam has games with their own launchers, although I might be speaking out of my ass here. The RuneScape client has its own launcher with no user interface. They would only need to have the wrapper on steam/epic, which would just need to be updated when there are major client updates in which you need to redownload it in the site (which does not happen often, right?). Not sure how they would handle revenue sharing. All RuneScape purchases are done not and through a weblink in RuneScape match, but they could likely mplement a steam shop.
Yep plenty of steam games have their own launchers but from what I have seen updates are still handled by them via steam rather than via the launcher itself. Meaning every week they'd be pushing updates via steam as well as through Apple, android and the present desktop client. Yeah that on its own is not a deal breaker but all Old School RuneScape Gold these little things accumulate very fast to make a simple proposal"put game on steam" into a pretty major project with loads of possible downsides.