Yeah both OSRS and RS3 have tons of “end-game” content.
Getting all 99’s is just a small portion of the content even though it can take a while, but getting all 99’s doesn’t suddenly make the game easier or leave you with nothing to do.
Bossing, PvP, minigames, (200m/pets as Eugene mentioned), achievement diaries, questing, mini-quests, etc. There’s a ton of content, RS3 especially is geared specifically towards “end-game” content and OSRS has been releasing content that gears towards that.
I put “end-game” in quotations because IMO RuneScape is not a game that actually has one - even if you were to spend 10,000+h getting 200m all and completionist, you could still enjoy things like bossing and mini-games the exact same way someone who isn’t maxed would. A players success on RuneScape comes from setting and achieving their own goals.
If you really feel like you need something to do get every treasure trail reward the game has to offer.