There's also the question of what qualifies as a "story arc" to various people.
I'm in a group that's going through Rime of the Frost Maiden. In one session we are hunting down a vicious moose, and in the next, we are booting kobolds out of a mine. These look and play like a mix of or random adventures, though in each one there's a tidbit that leads to the whole.
If your AP is a single dungeon crawl, yes, it will get tiresome. If your AP is built out of several different threads that combine to bring to an overall culmination, it may feel less constrained, as the PCs activities and concerns change several times over the short term
I don't know RotFM at all so I'll have to take your word for how it's designed.
That said, it can feel constraining if every adventure, no matter how diverse, still somehow ends up pointing at good ol' Bobby McNasty as being behind it all. There, I'd just want to get Bobby out of the way as fast as possible so as to get on to something that really is different, even just stand-alone adventures (which IMO are often the best anyway).
Princes of the Apocalypse, which I'm more familiar with, can suffer from this if not approached well: one adventure just leads (quite literally - they're joined together by passages!) to the next. In the long run, this would get really dull. Far better IMO to break PotA out into however many (15?) little adventures and place those independently around your setting, i.e. remove all the connecting passages; and then throw in some unrelated adventures or even have a second side-along story arc going to keep things fresh as you jumped back and forth.
The campaign might end up going something like, with each line being an adventure:
PotA 1
PotA 2
Other Arc 1
Stand-alone (with a minor tie-in to Other Arc)
PotA 3
Other Arc 2 but ties in with and leads directly to PotA 4
Other Arc 3
Stand-alone
PotA 5/Other Arc 4 - for one adventure the two arcs merge before again separating
Other Arc 5
etc.
And so you end up with at least a 25-35 adventure campaign* rather than just 15; and yes this would probably mean you'd have to slow down the character advancement rate, but IMO that's always a good thing to do anyway.
* - plus whatever in-game events etc. the players and-or characters want to follow up on later.