On the other hand, mono is 'open source' so it's kind of rubbish for Xamarin to be asking license fees when they have their own (pretty rubbish) competing game stack with monogame that they have a vested interest in (not commercially, but Xamarin developers work on it).
I find this sort of 'kind of open source but actually you have to pay us ahahahahaa~' thing pretty distasteful.
I find earning money on the work of others for free pretty distasteful.
Very disappointing that Unity haven't upgraded yet, hopefully they'll do so soon.
If the assets aren't covered, I think Unity could go LGPL in some way.
Person A: Invents something, fails to capitalize on it.
Person B: does marketing work, sells all the Monies!
Person A: -__- Oh that was clever. Actually, I want some of your money please.
Person B: Fuck off. <--- The correct response.
If you make something and want to sell it, sell it. If you make something and give it away for free, don't come crying when someone else sells it.
The 'dual license GPL' approach to software which is 'kind of free unless you sell it, in which case we'll negotiate some nice on going licensing fees' seems disingenuous to the spirit of open source (and specifically the GPL) to me.
Some tips: free doesn't mean "gratis", it comes from "freedom". Therefore it is normal that the GPL uses a strategy to avoid that derived works become proprietary ('proprietary' means: essentially "non-free" for the end-user, and "free" for the developer).
From your comment it seems that you confuse the fact that opensource is not something that benefits the developer; it is something that benefits the end user! Don't forget that.