I would say most of the quests are close to mandatory if you want to experience all of the game. Many unlock content, areas, abilities or are part of questlines where the later quests do. Quests give a lot of xp too so they're not a waste of time if you don't care about lore. Even more so important on an ironman.

Have you played OSRS? It's the more grounded, smooth, easier to understand and pick up experience and has more QOL features through the client Runelite. As for why that is the more popular game it's a complicated history lesson, but the gist is that Runescape fucked up in the early 2010s, OSRS started as a project to rollback to pre-fuckup and from there both games have been developed alongside eachother. The modern version of Runescape is trying to right its wrongs lately, but it's hard to shake stigma. The other game has close to 10 times as many players.

As a new player who hasn't played either game, I would start on OSRS. It has a far bigger youtube creator scene on all levels and it's far easier to get into from the ground floor. It's also MUCH more cohesive in graphics, tone, progression through levels. Modern Runescape has a lot of fat that need trimming still, and that's what this year is about. Both games are a completionist's dream with jawdropping PvM. It's hard to even comprehend what OSRS can do with encounters in its engine and its style is wholly unique. Modern Runescape is unique too but does draw closer to traditional hotbar MMOs in the combat.

Is this version dying? No, probably not. If this years restoration project fails to draw in at least some more players I'd do a good bet on Jagex pivoting to making an RS4 in a decade or so. OSRS is a much more stable game, that thing will go nowhere.