yeah, i can see that working. the game being free to play would probably attract a lot of new players, and they could even expand the microtransactions further if they wanted to be safer.
they could go the elder scrolls online route and convert the membership into a "runescape plus" kinda thing which gives you a slew of benefits every month, and maybe some small amount special content. i don't think they could switch to selling expansions at this point because people have enjoyed free updates for so long, but it kinda seems like they already understand that having more free content makes people more willing to put down money since they made some member quests f2p a while ago, and they've had some free weekends every now and then too.
frankly i feel like runescape has taken a pretty major back seat in the past few years, and i'd absolutely love to see it come back with something like this. was my first online game and i would love to come back to it, just don't wanna spend a monthly fee to do it