I think the matter is not that OSRS gold does not progress, it's that they have fundamentally broken player progression so interesting training systems do not matter. RS3 releases some pretty big updates every once in a while but how much of it is really used and appreciated?
Partly the issue is also perhaps that RS3 simply does not quite take thoughts far enough to make them something amazing. Think Shattered Worlds - people were excited about it but it just fell short of becoming something really amazing. Then Jagex only kinda goes on to another thing, which can be often half-baked. It's mostly crap mobs. It could have been very exciting if we got lots of new, exciting, powerful monsters to combat but instead we have dull crap. Like it was this close though,y'know?
And I believe OSRS has lower standards for what is an exciting update anyhow, most players are only doing their thing and if they don't get upgrades for a while it is not a big deal since they intend to grind for countless hours doing the same thing and they are pleased to do this. If they get some new way to invest hundreds of hours every once in a while it is a welcomed thing. On RS3 though it's kinda like... Eh. 99s are really easy, so in case a new training procedure isn't super good 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 battle stats so it's simple to add things that have a greater impact. Not to mention PVP is a thing, so some random weird weapon can become the focal point of an entire account.
There is also an understanding of RS3 not getting many updates, but it gets some pretty massive updates and when it will capture those massive upgrades it introduces a lot of things. Idk, there is just so many factors at play and it is barely"RS3 does not advance." OSRS just recently got the"Hallowed Sepulchre" agility activity however. And it's a large piece of skilling content comparable to Shifting Tombs, it probably is actually larger and more interesting than Tombs too.
Upping slayer to 120 even with what is there today still feels dull and lacking in content. The distinction is osrs and rs3 are two different games with buy RuneScape gold the same origin. Dead content structure and watertodt, really nice indeed lul.