I would dump so much gold into my apartment if it could keep upgrading it to be larger and larger.
Buy a backdoor that goes into an instanced room that just happens to think it's outside and lets me place outside items.
I would buy a door in a side wall with a staircase to a second floor.
I would buy upgrades to the apartment to make it larger and larger until it was medium sized.
I would buy upgrades to the stairs to have a lower floor.

If each of the upgrades cost more than the previous one so that the total footprint of an apartment that was a small was about the same price as a small, a medium cost about a medium, a mansion about the cost of a mansion, and the next upgrade 10 TIMES that cost.... people would probably still have gold to sink into it, and quite happilly.

Course it would require a lot of work to make the instance code able to support it.

On the other hand it would be a massive gold sink, even better than the gold mounts.