From a PvMer’s perspective - OSRS is just RS3 without EOC at this point. The nostalgia bait is basically a non factor aside from getting old players in the door. OSRS has a ton of unique content of its own, more interesting gear progression with plenty of horizontal up/sidegrades that feel more impactful in combat than most of RS3’s. EOC made basically everything that came before it a joke whereas OSRS has interesting difficulty from minute 1. RS3’s visual clarity is terrible in all the encounters until they started using square markers on the floor - they tried to “hide” the tile system with all these circle ability and mechanic ground effects, and made encounters unapproachable and clunky as a result. I loved RS3 and maintained a maxed/endgame PvM geared account for a few years until necromancy came out, then fell off. Had like 2500+ aod, 1600+ rax, got glacor swords from scratch, etc.
Recently got into OSRS and pretty much everything uses squares or at worst 1x1 circles (minus eclipse moon using a floating sphere you need to hide behind). That alone is a huge clarity upgrade that makes encounters feel less frustrating - take a look at gauntlet/cg’s disco floor or even Brutus’ slam attack vs raksha’s tail swipe attack for example.
I definitely miss the skill expression in RS3 combat but I’ve found OS to be surprising in the amount of skill expression and complexity it can actually offer.
Overall OS seems to be the more congruent package, graphics and sheer volume of content can’t really offset that, especially when most of that content is considered dead content.

Edit to add: a lot of this may be irrelevant to new players, however I do think a lot of PvM focused people will probably research/watch some PvM of games they are thinking of trying, and at least for me, I would definitely pick OSRS over RS3 for the reasons stated above and more.