How much official Lotr merch has there been over the years? For one, I think up until now it has been pretty scarce? Infact, I would say old merch selling for thousands should be an indicator of what possible hot Lotr items being produced now might be worth in the future.
Consider this, is there any POTC merch selling in the thousands? I honestly don't know but I would wager POTC barely scratches the surface of popularity when compared with LOTR.
Another factor going for LOTR over POTC is that LOTR is a huge part of nerd culture. While we may not like that word, nerds or whatever you call them are essentially who are going to be buying an EOL lego set years from now. They are the ones doing so with Star Wars sets right?
I don't think POTC is that big among nerds. I as a nerd myself definitely don't care that much about POTC. To me, it was just a disney movie that was ok. I know a girl who loves POTC because she thinks Depp is hot, but other than her, I don't know anyone who cares about them beyond the movies.
But I do know that nerds love fantasy( magic the gathering, dungeons and dragons, world of warcraft), and they love Lotr, they love Star Wars, they love Superheroes and they are the ones who will be the big collectors with high paying jobs who can afford a $400 Lego Set. And if there are nerds willing to pay thousands for old Lotr merch right now, there will probably be many more in future willing to thanks to the movies giving it just a little bit more nudge from huge cult following to actual popular culture (that which doesn't consist of just nerds alone). I say that because I honestly didn't know it existed till the movie came out. Of course I never played D&D either, which likely would have led me to read the books.
That or larping which would also probably had also led to discovering Lotr.
Basically I see it this way.
Was there a huge amount of people dressing up as Jack Sparrow for the premiere of Pirates movies?
not that I know of. Some people probably did but was it alot?
Was there a huge amount of people dressing up as Gandalf for the Lotr movies?
Yes. Just like for Star Wars and Superhero movies, people love to cosplay to these, because it is nerds having fun with other nerds.
Thus Lotr crosses the scope of what makes Star Wars and Superhero sets have great gains. I mean, we already know Star Wars sets can have huge gains. We already know Batman and Spiderman sets can have huge gains.
What do they have in common? Nerds
It should also be mentioned about Lego Castle themes. Why do people like them? Is it because Castles are just really cool? I argue the reason is because the fact that Castles are a part of Fantasy.
People who love D&D and Legos will want to make a Castle because they are related. This then has a hint of things to come for Lotr.