I'm surprised these aren't more popular given their classic feel and small footprint.
Could have, would have, should have ....
More metal and less plastic would have helped a lot.
Also, IBM made parts (eg keycaps, plates, mats) interchangeable across many models so that an ISO Model M can easily be converted to ANSI, for example. Why use all different keys and stabilizers on the M2?
I may still buy yet another one if I come across it at the right price.
Well to be honest, for keyboards, well-done plastic just makes more sense than metal. It's lighter (cheaper to ship and carry), completely corrosion resistant, has natural lubricative properties on moving parts, and arguably more durable against the type of stresses (light impact) keyboards may be subjected to (as in: plastic tends to flex and return to form, whereas most metals tend to stay deformed when bent). That being said, we all prefer things made out of solid metal - myself included - but that's not always a logically-derived preference. There's a reason Glock has a reputation as the most reliable handgun in the world, and its not because the frame is made out of metal.

As for the parts' un-interchangeability, I believe that's because IBM was going for a more "modern feel", wanting shorter keys and shorter throws - less old school "typewriter" feel and more practical for modern applications. (And, as a side-effect, gaming...) It is indeed a significant change from a standard Model M...
In terms of pure keyboarding feel my Model F certainly takes the cake, but as the logical choice for a daily driver this M2 seems hard to beat.
If you do decide to give the M2 another shot I wish you the best of luck finding one in good repair, that works, and is a great price.