There are so many holes in the argument I don't even know where to start from, lol.
Let's start saying - I don't know where you are located, but yours sounds like the typical statement from someone who lives in the US.
Yes US customers make up for the bigger market. Yes they can order Sideshow stuff, Prime1 stuff and Iron Studios stuff directly from Sideshow... at retail! Yay! Reward points! Reasonable shipping costs! Awesome... for them.
I know it might be shocking news and no disrespect intended here, but... the world is not northamerican-centric.
Don't get me wrong, if I were in the US, I wouldn't bother to do backflips and spend a ton of money on XM pieces too.
But, there's a whole world outside, made of high shipping costs, customs fees, VATs, every kind of stick that can reasonably be stuffed in a person's most sacred holes. A lot of asian countries where Sideshow, Prime1 and Iron Studios pieces are sold at the same markup you see for XM pieces in the US.
I perfectly know it... I live in Europe. Prime1 pieces in stores here retail at around 1400$. Some big 1:4 pieces by Sideshow like Lobo, Dark Rider, Sentinel, Galactus and such were more expensive than what I paid for many XM pieces.
And even if we're being pushed out of the market by increasing prices, increasing shipping costs, crappy conversion values and such, we still make up for a slice of the market. And companies can have their spot if they tap into these markets, smaller companies like XM that only need a few hundreds sold pieces to make a profit out of a project.
For the records, there's an european company called Tsume. They do manga/anime stuff, became more popular in the last year or two but they've been around for nearly a decade. They are now doing 1500-2000 edition sizes and their stuff goes sold out in an hour usually. I'm pretty sure that the vast majority of americans don't even know about them. Let's just say their "older grails" are worth thousands. Their Gaara statue goes for 3000-3500$ and people kill each other bidding on it when it pops on Ebay.
Yes, as much as the "definitive timeless grail of grails" Doom PF. Oh, and it's a 1:6 piece, did I mention that?
This is to say: there are markets outside of the US.
Moving on to the companies talks:
First of all, you'd make a serious mistake counting Sideshow out. They have been in the big scale (1:4) market more than anyone else, outlasted other companies, have the capital, experience, artists, knowledges. This alone should tell something. And they have adapted lately, their sculpt improved, the paint apps improved, they have been forced to increase the sale price of their pieces but in turn this gave a much better output. It's undeniable their recent releases are improved a lot in terms of paint and overall quality control. They are making better anatomies, bigger bases, more complex stuff.
Then, you're mistaking the actual merits of a statue with the status/perception of it. Iron Studios can pull the best movie Captain America statue (they did), yet it just doesn't have the legendary status of pieces like Gambit PF, Hulk PF and such. It's a matter of perception.
Which brings to the following point: Iron Studios does good stuff... but they're nowhere ahead anyone else. Yes their Captain America is a pretty much perfect rendition of the movie character, who has a fraction of a fraction of the popularity of its comic counterpart, but that's it. The XM Black Panther is head and shoulders better, arguably the Black Panther PF is better than their movie version. The XM Mark 7 is better than their Mark 43 and the Mark 6 Maquette, the level of paint app is simply superior.
Prime1? Yes they're doing great stuff, but mind it: their "perfect facial features" come from digital files of the videogames. They still require work to make 3D models productable, don't get me wrong, I'm not saying it's an effortless task on their side. But they're using stuff that has already been made with "perfect features" by someone else, which is fine, and they add value to it with great paint jobs. But I'm curious to see how they'll do on characters made entirely from scratch. And so far they have been hoarding licenses like crazy but I want to see the output.
Also, they are mostly doing 1:3 scale and videogame characters. Keep this in mind for later and for now let's just say that people expecting to pile up 40 1:3 pieces in a room like they do with 1:4 pieces will soon realize they don't have the space for it.
Hot Toys... well I won't even comment. Some people buy statues AND dolls but they're a vast minority, different market.
Not sure if you see the pattern here... the cake is being split up in many different slices with different flavours.
1:4 scale vs 1:3 scale, comic vs movie vs videogame (vs original stuff, recently), and so on. These are relevant differences. There are people that would spend 1000$ on a 1:4 comic piece and not even consider a 1:3 videogame piece. People that spend 1000$ on movie stuff and don't even care for comic pieces sold at 500$, and so on.
As long as there are people buying into each different slice, companies tapping into that specific flavour will make their profit and go on doing their thing.