Anything short of a perpetual, binding agreement to never do this type of shit again is nothing more than "we're sorry if being awesome made you idiots mad". Get fucked.
Wasn't it just six months ago or so that Dungeons & Dragons was going through a similar debacle? That they can change the terms of the license post release is insane.
Ahemm as I understand the previously license did have a "we don't change this license on you" clause, which they removed shortly before this change. As I understand there is atleast possibility, that some existing customer developers might upon being pressed take unity to court over "you said you wouldn't change the license fundamentally without our consent, we had a deal".
What the exact language of that clause and would it hold in court challenge, I don't know. Just heard one interviewed developer say something to affect of "hey they did have we don't change the deal clause, which they sneak removed on pretty recent license update".
I atleast as business would not agree to deal of "yeah we have a deal, except this deal allows us to change the deal however we want".
It might mean having to do time limited or project limited deals, since on otherhand no provider would agree to "we have no room to change deal ever". I would atleast in case of say game development expect clause for example "any fundamental license change must have 2 year announcement time for existing customer." Such clauses are very common in "on-going basis contracts and deals". Heck international treaties use such clauses "If you want to leave this treaty, you must give other treaty parties 1/2/3/5 year notice and for the duration of that notice period you are still bound by the treaty".
So I would guess: If this ends ugly, there will be lawsuits over was the license change contractually legal, were the possibble change notices clear enough upon the main change being in itself legal and for example was some jurisdictions fair and good behavior clauses of national contract law itself violated. Was enough notice time given etc. Since one cant make any contrac or contract change whatever one likes, business contracts are always subservient to local contract law regulating what can be agreed, how and what amounts to stuff like informed consent, how contract terms can be changed and regulation on prohibition of underhanded or deceptive business practices.