Most mods out there leverage properties produced by other modders. This is because talent takes all kinds of forms. A person who makes gorgeous models may be shit at level design, or may be shit at story telling, or shit at voice acting, or shit at [Insert FOO].
The mod community gets around these individual failings by allowing "Good Story Guy" to leverage "Good script guy" and "Good model guy" and "Good level design guy" to create a mod that tells his epic story, and does so with quality components.
The same is likewise true for good model guy-- who can show off his awesome models with a mod that is worth playing, because it has good story guy's story-- etc...
What happens, fundamentally, when people start planting the :"I WANNA BE PAID!!" flag?
Several things. The obvious one, to me, is this:
In order to successfully monetize a property, then that property must be licensed, and actively policed and controlled. That means that if Good Model Guy says "Hold up, My models are so clearly awesome, that you have to pay me $BAR percentage of your gross if you sell your mod, and it features my models." Suddenly, Good Story Guy can no longer get his epic story out in a presentable container. His talent dies on the vine, because once he has done the math, and computed all the nickles and dimes he has to pay everyone to satisfy all their egos (which is really what this is about.) he either has nothing left, or worse, is actually in the hole, financially. This is simply due to all the overhead costs needed to properly attempt to license the properties, the costs of utilizing an IP lawyer to assure legitimacy of the licenses, etc. The ability of Good Story Guy to shine vaporize.
The same is true for Good Model Guy, who now has to license the level design skills of Good Level Design Guy, and the story of Good Story Guy, etc.
To me, wishing to be able to monetize your hobby/labor of love is like wishing that you had a magical castle. Boy, it sure would be nice to have, but when you look into it, you find that it just isn't really possible, and still have the community. You take what was once something with practically no barrier to entry other than your own talent that you can bring to the table, and overnight, you end up with a byzantine network of licenses so complex that you WILL need a lawyer to keep it all straight.
So, let me ask you-- Can you afford the services of a lawyer? All the time?
That's what going outside the "handouts" model *WILL* necessitate.
Either to help you draft your license to that it is sane and useful by other people (so you dont shoot yourself in the food), and just to make sure that any project that you arent the 100% rights holder to has properly licensed all of the properties that it leverages.
Paid mods outside of the donations-based model are simply, and fundamentally incompatible with the foundational bedrock of the mod community: The ability to leverage one's own talents with the combined talent pool of all other modders, to make something new and awesome, and do so without excessive barrier to entry.
At "best", "License based" mods would splinter the community into closely knit consortia, where you have "elite" (with HUGE barrier to entry) individuals that routinely license each other's properties at reduced, or even free rates, to produce community mods that they then share the proceeds from, based on some internal agreements. Such pools will stagnate, since no new blood can easily enter (because they cant showcase their own talent easily, due to the barrier to entry caused by the licensing model itself) and so such communities are doomed to slow death from entropy. (People change careers, get married and or have kids, anything that takes them away from their group, without ready replacements to take over.)
So, as harsh as it sounds, I equate "I WANNA BE PAID!" with "I WANT A MAGIC CASTLE!"