I am going to answer this for myself, I think that at least some of what I say applies to other developers but I certainly don't speak for all of them.
Unfortunately unless the income can be enough to consistently sustain someone than payment for work on tiddlywiki can't be much more than a nice gesture. For me tiddlywiki is a genuinely interesting bit of technology and I like playing with it, so that is why I do it. There have been some people who have donated to me and while I am very grateful for that, it has totaled about 3 meals at nice cafes here in Paris. How happy getting that made me is far out of proportion to the amount given, but the effect on what I can do is small. Unfortunately because I need to pay rent and eat these donations can't have a big effect on how much work I can put into tiddlywiki unless they are enough to cover all my expenses since I couldn't quit only 10% of my job. So as much as I would love to be able to get paid to work on tiddlywiki I don't think I could accept much more than people have given me because I couldn't make any sort of guarantee that I would be able to actually do more work.
For someone in my position at least there isn't really a way to pay a little bit for a little bit more development. I doubt I am ever going to say no to donations, but I don't really expect them and the amount that would be required for me to be able to put more time into tiddlywiki is probably impractical.