very very cheap, cash or miles speaking. fearing that to slap on a "uniform upgrade cost per leg" would cost the airline dearly as some may want to pay much more for an upgrade as opposed to others.
anyways, most people who owns some money is not stupid - and had they wanted to pay they would have bought a ticket at the higher class in the first place. I dont really think this would end up helping them generate heaps amount of income. it's way easier had they allow paid upgrade at checkin for all ticket holders. - seriously, i dont get it. people get O class tickets, but why dont you let them upgrade to J by paying the full fare difference (just a transparent fixed cost)?
anyway, if you want to see this as a way for CX bashing, I dont think it will work very well. I dont see why I cant book J/F for Japan domestic flights either as a tag-on. seems like market segmentation to me.
This is a bad idea and I can imagine most of the best CX customers would be highly annoyed by it.
But if it does get implemented, there should at least be a component that favors higher MPO members. For example, make it only available to MPO GO or higher (of course, I'm a GO so I would say that). Or offer it to elites at a (reasonable) flat rate first, then open it to bidding by everyone else.
If it's wide open to any steerage passenger on the lowest fare, and they can bid their way up past the elites for a few bucks, that would be a recipe for revolt.