The thing is that there are lots and lots of different side things, and you can pick from any of them to get what little extra XP you need. That's not making any of them individually mandatory. You need to do something but the choice of what is up to you. If the MSQ alone took you all the way, that wouldn't leave the option of doing much else without ruining your main story. Most players want to do at least a bit of the other things the game has to offer.
I actually find that, for my playstyle (which includes wanting to do most of those side things), they've gone a bit too far the other direction, with XP coming so fast that I'll drastically over-level content if I'm not careful. So on my latest alt I'm leveling multiple classes in tandem, because I need to alternate between them in order to avoid over-leveling so much. (Admittedly, that's more of an issue during the first 20 or 30 levels than it is later, when leveling starts slowing down. But over-leveling issues can still happen later if, for instance, you want to keep re-running a dungeon until you get its best loot.)
There is a level gap in the main story between level 46 and 49, but there are two optional dungeons around there (Dzemael Darkhold at 44 and Aurum Vale at 47). A run through each of those should cover the gap. (And if you get lucky on loot drops, Dzemael Darkhold can drop the BiS shirts for levels 44-49 for DoM or damage dealer DoW.) Plus, the ARR beast tribe quests open up between levels 41 - 44, so those will be available and not likely maxed out yet.