OSRS is pretty good at giving you a feeling of accomplishment when you complete things after a decently long grind since almost everything you do in-game in terms of skills&unlocks helps you work towards achieving everything there is to do in the game.
There's also that everything you achieved in-game will still hold value and be there 5-10 years later which is something you don't get a lot in games anymore nowadays.
Most games are hyped up for a while and once that dies down everything you did in those games is basically worthless since everyone moves onto the next new thing.

As for end-game stuff I can understand that once you max out the basics you will probably feel a bit of a void but that's the point where you have to start making your own goals instead of relying on things like skills and achievement diaries which tell you what to get and do.
Think of things like 200M exp in all skills, collecting all pets, collecting all rare/useful items, getting all minigame items, etc.