I think that when it comes to relatively simple plug-and-play solutions, modern Stax headphones are competitive price/performance-wise at any price-point with most mainstream headphone hi-fi gear - or are just outright better. What that means is that the SR-001 is competitive with any amped portable system for the money (better, IMO), the 2020/2050 is definitely competitive with or better than most entry-level dynamic headphone/stationary amp combos, and so on up to the O2, which is easily competitive with top-tier balanced dynamic offerings.
But, if you dig deeper, and start to look at vintage headphones and various undiscovered gems, I find that a lot of modern Stax headphones underperform for the price, until you get to the O2 together with aftermarket boutique amps. There, you'll get your money's worth, but it will be one heck of a lot of money.
What I mean is, sure, the SR-404/SRM-323 system will easily match most popular dynamic headphone/amp combos that we readily recommend without giving them a second though, and will probably beat most of them too. And, it's a bloody shame that we don't recommend it for general purposes more often, instead of some of the crap that we generally throw around at unsuspecting members. But, I had a vintage K340, costing $200, out of a vintage tube amp, costing another $200, that blew the SR-404 with the SRM-007t amp away. That's, what, a $400 system against a $1500-ish system? For the same money as that Stax setup, I ran an HD650 off a push-pull speaker amp, and it was simply in a different universe sonically. The only thing that compared to it was the O2.
So, while I share a certain planar enthusiasm (especially when it comes to the SR-001/003) I think that, if you're willing to put in the effort - and especially DIY effort - you can come up with a better dynamic rig for the money than a modern electrostatic one, until you get to the very high end. There, IMO electrostatics rule supreme.
Note, of course, that I say "modern electrostatic" instead of just "electrostatic." I have no experience with vintage 'stats. It's entirely possible that the same $400 will net me a system that would blow my K340 rig away.
BUT! That doesn't mean that you shouldn't look into modern Stax setups, since, as I've said, they're easily as good as most dynamic setups that are commercially available, and sound a lot more attractive to my ears.
In terms of what's driving people away from Stax - I think that this is similar to what's driving people away from vinyl. With a Stax rig, you need an AC outlet, a dedicated amp (no matter how inexpensive it is), and a pretty good source (for the entry level systems anyway, the high end needs a monster source). None of this has to be expensive, but it all takes time and effort to put together, and more importantly, it's all tied to a stationary location. So, when you listen to these systems, you pretty much have to sit and listen, dedicating time that, in our busy lifestyles, could be spent on the move. When you buy something like a D2000, sure, it doesn't sound as good as even a well-driven SR-003, but it can be used anywhere, out of nearly anything, and it doesn't restrict what you want to do when you're listening to it.
In other words, Stax, by nature, are much more suited to being a "main rig." And a casual newcomer to the forums isn't interested in a main rig. He wants a general all-round solution, and other than the 001, Stax doesn't really offer that. Then, once you put some money into dynamic gear, it's harder to dump it all and go into something different rather than go along your existing path.
It's the same thing with vinyl - you have to put a record on, and then you basically have to sit down and listen to it. It doesn't have the convenience of an mp3 player and a pair of portable headphones, or a hard-drive full of mp3s. It doesn't fit into the modern lifestyle as well, no matter how much better it can sound.
Still, don't get me wrong, I love 'stats. I didn't start out in dynamics; I grew up on 'stats and that's my "default" sound. Dynamics were a fun diversion for a while, but I'm over that now. That's just my thoughts after several years on both sides of the fence.
P.S. In terms of electrostatics being only suited to classical and acoustic genres - that's BS. Even more BS is that 'stats can't do bass. Sure, 'stats on average have less bass impact than dynamics, but saying that 'stats can't do bass is like saying that dynamics can't do speed or detail. You have to hear a (well-fitted, this is critical) Qualia 010 to belive how fast and detailed (and just how plain @#$&ing good) it can be. You also have to hear a simple 003 out of an SRD-7 Pro to hear how impactful it can be. Properly driven, it has enough bass and impact to shatter your skull.
I think 'stats can do more genres right than dynamics can. Dynamics struggle more on fast and dense music than electrostats do on bass-heavy music.
P.P.S. In terms of Stax lacking build quality - that's not just BS, that's just insane. How many working SR-5's, SR-X's, Lambdas, Sigmas, whatever have you, do we still have, in many cases 40 years after their initial release? They may not look good, but I have never, ever seen another company put as much thought into the construction of its headphones as Stax goes.
These headphones are precision instruments. That means that you have to treat them right, and you shouldn't abuse them in the same way you can abuse cheap dynamics. But, if you do treat them right, they will literally last you your lifetime, and try saying that about another headphone brand.
P.P.P.S. By "mainstream" I don't mean Bose. I mean stuff that's popular in this community, and others like it.
@#$& me, another essay. K cat, back to work.