As long as more gold is grinded than spent, ad infinitum.
But incresing the upkeep would slow it down.
I do not think that increasing the upkeep would increase the gold spent upkeeping tbh. If you increase upkeep, people will react and wear less equipment, possibly even reducing the total amount of gold lost due to upkeep in the end.
I think we need to offer new possibilities in order to slow down the inflation. Maybe 4th heirloom levels that can only be crafted from the +3 version with a shitload of gold (like... 5 million ?). Of course as the inflation continues, 5 million will probably become not so much, so we will have to invent yet another thing for players to spend their gold. But that's how inflation is regulated in all the games I know of. Another good idea is fiddling with the marketplace fees. By increasing them, you make people that use the marketplace pay for a part of the inflation. Even better, the amount of gold they loose through it depends on how extensively they use the marketplace, so this actually means that it directly takes most of it's money out of the trader's profit.
Finally, there should be more incentives towards retirement. Pretending that heirloom prices go up because the few very rich people that together make for a marginal part of the whole money in the game decide to spend a lot isn't just going to be true because people say it. Prices go up because ,in this case a the same time, the offer goes down because people stop retiring, and the demand goes up because the same people that stopped offering heirlooms to the market now want to buy some for themselves ! Thing is, the heirloom buyers that still wish to buy with current "crazy" prices have the majority of the money in their hands, not because they are very rich, but because they are many.
