Regardless of whether = or : is the better syntax, this change implies such a tremendously huge amount of churn that I consider it a complete non-starter.
For comparison, when I wrote the dyn trait RFC, I always felt that was just about the maximum amount of churn we could possibly get away with, and hopefully the churniest change we’re ever going to do after 1.0. And that was a case where the existing syntax was so deeply confusing that people often had no idea they were operating on trait objects rather than traits. Even if you don’t like today’s struct syntax, I don’t think anyone’s found :s so confusing that they failed to understand they were initializing a struct.