Here, the problem with paid content is not whether it's given away for free in the end, but rather in who gets a
say in
what the content is. Patron-type supporters usually get some rather minor say, your version comes potentially with a notarised and presumably enforceable contract.
Now, it may or may not be a significant problem, depending on the details, but I kinda felt the urge to rant a little, so...
In my experience, the problem with paid modding is threefold:
- It prioritises one person's desires over an entire fan base, one that is usually much more relevant to the game/mod. The 'whale' problem, so to speak. Not really a modding example, but some really egregious examples that come to mind are Drevan from Pathfinder: Kingmaker and all the 'purple prose' backer characters in the first Pillars of Eternity.
- It provides perverse incentives to the modder, blurring the line between hobby and work. On one end of the spectrum, the ultimate worst end of this path are the Patreon grifters who make a dozen 'updates' per year and are constantly 'ill', 'depressed' and 'on vacation'. The other end is making modding too much like work, to the point it is no longer fun and the modder retires and goes on to find more relaxing hobbies.
- Competition and drama within the modding scene. We've had our own version with Hobbes and the Terrain Pack that got used in Piratez, which was taking (small) donations. Perhaps the most famous example is the fiasco when Nexus made their 'paid modding' system. They've walked it back somewhat, and it was not the healthiest scene in the first place, but still.
And, well, usually the reason people don't take paid commissions is because they've already got more things than they will ever be able to tackle on their todo list, and a commission is likely not to align very well with their overall vision. If the stars align and the commission is something they really wanted to do in the first place, that's where this kind of proposal can work out reasonably well.
Case in point, I gather from Solarius' reply that just replacing Cydonia with another mission is not what his end goal is. So at best this will be a placeholder mission or missions that gets replaced at some point, at worst it'll be creative self-sabotage. In either case, it'll be 'wasted' time from the perspective of realising their own vision.
Also, what Solarius said. Modding has been walking the line between 'authorised by the IP holders' and 'blatant piracy' a lot, and most genuinely interesting mods I know are guilty of intellectual theft to some degree of another. So, as a sort of penance, modders generally try not to gather more coals on their heads by also asking for payment.
Finally, your example of getting paid to write features for open-source projects in the hopes that the community will maintain those for free doesn't really sound as A-OK as you're trying to picture it. Essentially, it seems that you and your client were hoping to cruise
forever on other people's voluntary work in return for a one-time contribution. Perhaps there's more nuance to it and it was not quite as one-sided as that, but this seems to be the essence of it.
In the end, these might or might not be insurmountable problems in a specific case, like this proposal here, but there are enough issues to make any experienced modder wary of taking such an offer just on principle.
P.S. There are paid tabletop GMs, and opinion on whether they're worth it is just as divided as the opinion on paid modding, if not more so.