Which still doesn't really effect anything.
a) If the developer wants to continue sales,
they must fix their mod to work with future updates.
This gives them incentive to keep it up to date.
b) It's an accepted and known risk that updates
might break the mod at time of purchase.
I don't use Steam but I assume it lacks Version Control - so you wouldn't have the option to download an outdated version of a game for mod compatibility. [0]
The ability to sell a mod could incentivize people who would otherwise not create mods.
So here are the two scenarios:
1) No sales = less content created; same issue of future incompatibility
2) Sales = more content created; same issue of future incompatibility.
Those who do not wish to risk a mod becoming incompatibility (and unfixably so) could simply not buy any mods that are for sale, choosing to stick with 'free' mods.
The suggested donation-model (optional payments) is how I think most media should run regardless. It seems theres a fair amount of support for a "donation button" instead of the sale model.
[0] Based on nothing but posts from 4 years ago and a quick Google search and the fact that future incompatibility is an apparent issue.