The grievances have been simmering, but anecdotally at least I haven't known a single developer who is unhappy about it enough to actually switch. Also programming languages/frameworks/platforms are highly sticky. Apple was very smart to make their language/framework non-cross platform (generally speaking. By that I mean, try finding jobs doing Android or even web development in Objective C or Swift. I know it's possible but it's not very practical) and unique to their system.

At the end of the day, developers are users first and foremost, and as users they are thrilled with Apple. Things would have to get pretty damn bad before Apple would need to worry about losing developers.

If developer grievances about Apple have been simmering for years, perhaps they can simmer longer.

Apple will make the change that they're forced to make. Developers aren't forcing anything at the moment, but what exactly are they being forced to do by the EU? That's what's being learned.

As a long time Apple fan myself I'm certain, Apple fans have never been happy. :)

Enthusiastic small developers existed while there was a profitable market. Third party app stores allow piracy and make being a small developer much tougher, I doubt there will be much enthusiasm going ahead

What is happening seems to benefit mostly massive competitors like Epic or Amazon.