Thanks for this - these are great points.
The main reason for having multiple drivers is that the balanced armature drivers used in a lot of (not all) IEMS have narrow ranges of frequencies that they can reproduce efficiently. They fall off quite steeply outside of those ranges. Therefore you need to either have lots of them (expensive to produce) to cover a lot of sonic ground, or you have fewer (less expensive to produce) and make compromises about what frequencies will have strong response and which don't.
Dynamic drivers, on the other hand, can cover much wider ranges of frequencies efficiently, and so you don't need lots of them to cover a lot of sonic ground, especially (as @dkelley noted) if they're close to the ear and don't need t worry about directionality and a number of other factors. There are high-end earbuds that use dynamics either in a single-driver configuration or multiple, or even combined with balanced armatures. There are also lower-end earbuds that use dynamics, such as the Shure se215 or, of course, most consumer earbuds including as Apple's earpods or the like. The shure se215 isn't a great earbud, but it's not simply because it is single-driver.
So if you're talking about balanced armatures, more
might be better, but design and intended use makes a huge difference.
@s0c9 we've had (hopefully!) healthy discussions in the past about this but I can't agree with a blanket recommendation of triple drivers or more, without recommending that the buyer look hard at what the intended sonic profile is and what design compromises you're willing to make. Not all 3-driver earbuds are bass/mid/treble, and not all 3-driver earbuds have a big bass driver. Not all 2-driver earbuds lack bass - there are some satisifed Alclair dual XB users in this thread, including me. The sonic compromises made for that model work for me - as does the price.
Finally, to all of those who just throw out a recommendation of 64 audio or JH or other >$1000 earbuds...there's a lot more out there at more accessible price points that might be more appropriate to suggest to someone who's looking at their first upgrade from se215s! Or...just ask: hey @pshep68, what's your budget?