Runescape 3 is much more fun and engaging than OldSchool Runescape.
OSRS combat so boring and generic, with RS3 it's more engaging with better mechanics and the use of abilities which makes combat and PvM leagues better than OSRS because we have more room when it comes to designing boss mechanics given our advanced combat mechanics.
No argument for why Runescape 3 is bad exists, besides false information about 'MTX' (microtransactions). Runescape 3 is NOT pay2win, MTX allows you to level faster yes, but you can easily max every skill without it (which by definition voids the pay2win argument).
You don't win by having 99s anymore either, even 120, hiscores were filled long before MTX came around so no one will get a good ranking on hiscores just because of MTX. PvM, questing, completionist cape, etc, can not be achieved or done with MTX - only by playing the game.
Fact is Runescape 3 is much more advanced both graphically and mechanically, just comes down to personal preference.
P.S no one cares about others online achievements even on OSRS, which is the main reason people dislike 'MTX' cause it voids their achievements lol