I think that the matter isn't that RS3 does not progress, it's that they've basically broken player development so interesting training methods don't matter. RS3 releases some pretty big updates every once in a while but how much it is really appreciated and RS gold used?
Partly the matter is also perhaps that RS3 simply does not quite take thoughts far enough to make them something amazing. Think Shattered Worlds - folks were excited about it but it just fell short of being something really amazing. Subsequently Jagex just kinda goes to the next thing, which is also often half-baked. It's mostly crap mobs. It could have been really exciting if we have a lot of new, exciting, powerful monsters to combat but we got boring crap. Like it had been this close though,y'understand?
And I think OSRS has reduced standards for what's an exciting upgrade anyhow, many players are just doing their thing and if they don't get updates for a little while it is not a big deal because they mean to grind for hundreds of hours doing the exact same thing and they are happy to do that. On RS3 although it's kinda like... Eh. 99s are really easy, so in case a new training method isn't super great money and/or AFK what's even the point to most people? Similar story with items, OSRS players actually have a reasonable desire for lower grade weaponry because people are not all level 99 in their combat stats so that it's easy to add things with a larger effect. Not to mention PVP is something, so some arbitrary weird weapon could become the focus of an entire account.
There's also a perception of RS3 not having many upgrades, but it will get some fairly massive updates and if it will get those massive upgrades it introduces a lot of things. Idk, there is so many factors at play and it's barely"RS3 doesn't progress." And it's a sizable piece of skilling content comparable to Changing Tombs, it likely is really larger and more interesting than Tombs too.
Upping slayer to 120 even with what's there now still feels dull and lacking in content. The difference is osrs and rs3 are two different games with cheap OSRS gold the same origin. Dead content structure and watertodt, really nice really lul.