its not going to affect the odds of a 72 hour upgrade unless they increase the amount of seats open for U class in advance

To which I dont think theyve done that and probably dont plan to. If anything the U bucket keeps shrinking in numbers. Makes no difference if an MVP buys it as an S fare or if a gold buys it as an H fare or a 75K buys a Y fare

It will also entice more mvp to buy a non refundable upgradeable fare and then the airline has your money in perpetuity. I think thats part of what they are trying to do here.