And yes, the particulars of this license seem more than reasonable from the standpoint of the threshold of needing to pay.
I should maybe point out that at one point, ImageSharp wanted Microsoft to stop their (still open source) graphics development and help contribute to ImageSharp.
I think being upset or frustrated that one has lost access to something useful that one also invested very little in is a fair/natural reaction (the free lunch is over), but I also think it would be unfair to direct that frustration at the people who were providing that resource to begin with. It also seems unfair when the resource, as it stands right now, continues to be available in its current state; one only loses out on additional, future value, which I would argue one has no real claim to anyway (in general).
Your comment didn't say that it is fair [to direct..] (so take this comment as a further qualification if anything), but it seems to be one of the prevailing attitudes about this kind of thing, and where (at least) I see the entitlement coming through.