with emphasis on the they

OPM is a very big factor here. Not that there is anything wrong with that.

Also, sometimes, in for a pound.....

For instance, one that has the means who regularly buys M+ for €2500 (with the intention to upgrade), but now is unable to confirm a few upgrades decides to just purchase the Z for €3400.

But, in my experience, most OPM situations are either BusinessElite or a fare class that cannot regularly upgrade (new, Global Upgrades, notwithstanding). Often those required to book coach are required to book lowest fare, and those fares are often in classes below M+. Even with ticket change fees, discount fares often are most cost effective (even with a change when necessary, and change fee) for these flyers.