If you have to tell something to the users who try to donate to Anki, you can tell them that some add-on authors do accept donations. For one, if I register as a donator at such a place, it won't be long before I unregister, so it will be helpful if I can donate soon rather than waiting for years.
@Aleksej: I love many add-ons but in the end you can live very well without them. Damien doesn't directly profit if add-on authors get more money. The most important thing about the Anki ecosystem is that the main developer gets money.
I would say this: People who want to support Anki should buy the ios app. If they already have bought it or don't have an ios device they could buy the app for a friend or family member who has an ios device: Spaced repetition is extremely useful but still many people don't use it. So the person who buys AnkiMobile not only helps the Anki developer but maybe even more their family or friend. Also you are more likely to stick to an app if your friends also use it.
This is not just my idea but has been repeatedly brought up by different users in the anki subreddit.
ijg: while of course I'd appreciate it if people could support Anki by buying AnkiMobile, I do not have any problem with add-on authors asking people to donate to them - they deserve to be compensated for their efforts as well.