At the moment I use following string in my scalacOptions:
"-Wconf:msg=Alphanumeric method .* is not declared infix:s",
This silences all warnings about infix methods.
Using this in SBT makes infix acceptable in my project anytime anyone sees fit.
As long as this work, I have no real problems with this feature, as I am not forced to use it. I would prefer the silencing to be less drastic (more similar to using other optional language features, like dynamics), but practically speaking this works fine.
(I hope this post will nor result in further tightening of ropes, which would really require me to list methods one by one or something like that).