I am happy to disregard map literals for the time being. They were anyway only a marginal part of the proposal.
About tooling concerns: Isn’t that a bit overblown? It’s a trivial syntax change. Took me about 5 minutes to change the grammar and the parser to support it. I agree that recent changes to givens and named tuples did pose deeper tooling challenges. But this?
One can also see it the other way. Collection literals would be a big help in approachable DSLs for tooling. For instance, I was told that they would be great for simplifying Mill build scripts. So a more approachable syntax also leads to more approachable tools. And this is the kind of simplification that matters for these tools. HKT or not, who cares? But a straightforward way to define a bunch of things that does not mention implementation details is needed in lots of places.
About the the concerns of learnability: It’s evidently not a problem in a dozen other languages. Almost every future programmer will come from Python where collection literals are everywhere. These future programmers will be pleased if they find the same syntax in Scala. They will be put off if it’s absent because we insist that collection literals are too hard to learn.