Decades of experience have taught me that gold is never a true motivator, it is instead an intermediary currency for what the players may really want. If you build mechanical effects for gold expenditure that lead to XP (a problem in and of itself if you are using a milestone system to level up, but lets assume not) then some players will game that mechanic for the XP rather than the engagement experience.
I think it would be a useful optional rule for a group that needs it or sees some value in it, but I think it would only be useful in that way. I'd never adopt this for my game table, where my main goal is to get the players invested in the story rather than the loot. The story elements should not hinge on wealth expenditure whenever possible. Hell....my current campaign, as an example, is 10 levels in with the only endgame reward being land and title to the PCs for their deeds. The material wealth has been infrequent, but the boons and gains through the tale telling have been far greater rewards than any coin.