If you’re happy (or happier) with the single-driver model, that’s great.
There’s a number of them I like too … though I replaced my original (20 year old) ER4-S with the ER4-XR as the original was a bit too lean and I tend to use that it in situations where useful EQ either isn’t available at all, or causes more problems than it solves.
Still can’t get around the distortion factor with full-range single-driver BAs today, though. So, again, you’re either limiting their bandwidth, or accepting more distortion. Which is really just my response to why companies bother with multi-driver BAs still (your original question). That one segment of the community is happy with single-drivers (and their benefits), while another wants the opposite set of trade-offs is nothing new.
As a rule I don’t compare IEMs to full-size headphones; they’re totally separate use cases for me.
My favorite architecture for iems is the quadbrid design. Dynamic driver for bass, balanced armature for mids, electrostatic drivers for highs and ultra highs, and finally bone conduction driver for adding more character across the sound spectrum. Two iems that incorporate all this technology really well are the UM mest MK II and UM Indigo. I own both, and they really shine with all music genres. The Noble Audio Kublai Khan also employs this Quadbrid design. I have not had a chance to hear them, they have really good reviews. I also own the EE Odin and it does really well for a Tribrid. I find all BA driver iems to be unable to deliver strong bass response like the Dynamic Driver iems.
Is there any practical way to check the polarity of the drivers in a multi-driver iem without actually cracking the thing open?
A while ago I bought a TRN90 (4BA/1DD) and it sounded off out of the box. The bass was pulled off to one side - the whole stage was somewhat offset TBH. Experimenting, I flipped the iem cable on one side (kind of a pain since everybody wants to make iem cables with earhooks now) and that centered everything so it kinda of sounds normal. I say kinda because I have nagging doubts about whether everything was out of phase or just the DD.
I bought the iem on sale off AE, and after some back and forth with the seller (they wanted a video, like a video could demonstrate the problem) I threw in the towel and decided to eat the loss and call it a learning experience.