An alternative to revshare? Thread

I'm a software developer developing a game, and I've been considering outsourcing some 3D art creation for some time.
I thought about revshare but there are several reasons why I don't think it's a good choice for me. The first is that the artist has no way of ascertaining how likely it is for the game to get finished and make a profit. Second it's a bit problematic with a set percentage because it kinda hinders growth of a game unless we're talking partners who are in it for the long run, and are willing and able to commit themselves to continue adding value as the total value of the game grows.
And it also creates unnecessary issues with trust - or adds complexity with audits etc.

So I've been thinking of another model, inspired by how some artists have chosen to also have indie-friendly pricing.
The basic idea is that you initially pay a low price, but this price only grants you an exclusive right to the work for a limited amount of time, with an option to upgrade to unlimited exclusive rights for an amount that has been agreed upon beforehand. This upgrade amount + the initial price should be substantially larger than if the work had just been commissioned normally.

At the end of the exclusive right period 1 of 2 things could happen:
1 - You lose all rights to use the work. The artist is free to either sell the exclusive rights to someone else or sell it on asset stores.
2 - You only lose exclusitivity. The artist is then able to sell it on asset stores.

The first option is obviously the best deal for the artist, and the second option might not really be much better than the first for the game developer.

Example: Let's say the "real" value of some piece of work is $1000. Then the initial price could be set to $500, and the upgrade price to $800, for a total of $1300 paid if the upgrade option is excercised - a premium of 30%.

A major drawback of course is that this will only truly work if the work in question is general enough to be marketable to others in case an upgrade isn't made. If not then the only value for the artist is the incitement it creates to upgrade.

Also it will only really work if no third parties have any claims on the IP in question. This means if the game developer supplies concept art and is also outsourcing this then he/she must secure full transferable rights before handing it to the 3D artist.

Has anyone tried similar models? Are there some drawbacks I haven't thought of or does it just sound like a terrible idea? What would reasonable relative initial prices / premiums be?