It may not be a new market, but the market has changed.
The potential customer pool has grown with the in-game marketplace. Gone are the days where you must look for, then obtain addons from various sites. The sim also handles installation, updates and uninstallations (or it will do so when the marketplace is improved).
Add to that the launch on Game Pass and on Xbox makes the sim more accessible from a cost, and device standpoint. This has the potential to greatly increase the potential customer base.
It is probably too early to see what impact these changes have on sales, but it is possible prices could come down whilst allowing developers to break even or make more money.