It's a question of balance, people don't pay extra for fixes because they've already paid for them when they bought the game, honestly I would have preferred that the game's development ended before they butchered their engine with Gathering Storm and then added more and more content over a now unstable base...The modes are like scenarios, part of the paid content that keeps development going. I only like a few of the modes just like I've only played a few of the scenarios. No big deal.
The bug fix patches you're referring to are completely different. People don't pay extra for fixes. The premium content keeps devs invloved. There were a number of free updates that came out along with the various NFP releases including the April patch. The civs and modes don't take time away from bug fixes. They facilitate it. Capitalism Baby!
The modes take so much less effort than the various scenarios they've added but I've never really seen the same complaints about them. I'm going to go out on a limb and bet more players are using the modes than ever played the scenarios.
I also disagree on the "modes take so much less effort than the various scenario", from my modder's perspective it's the opposite, but on the other hand, I can agree that modes generate much more sales than scenarios.
I can also understand that the NFP was a way to keep/protect the artists team by generating income before working on a new projects.
But again, balance, seems they have not a lot of coders (which also is an argument to say that the development cost of the NFP wasn't at the detriment of bug fixing, as it was mostly done by people who don't fix bugs), or they've moved them to the next project first, which is logical, but IMO the game really needed more fixes before.