Again, the intention was to
have a discussion about doing so early in 7.12.
* Keeping foldr, etc. in Data.List in 7.10 monomorphic in the face of a generalized Prelude would break an enormous amount of code. It would force users who currently do not use qualified imports to do so and increase conflicts over all. As it is, even Neil and Lennart on the agreed on a call the other day that the Foldable/Traversable changes break almost no code.
* Keeping those combinators in there and generalized indefinitely breaks basically no code, but leads to flare ups like the current one where folks look at the illogical-seeming intermediate state and get upset.
* Removing those generalized, redundant combinators after a deprecation cycle gives a path by which in the annoyance at the state of affairs in the second option can fade, admittedly at the expense that there is no place for the monomorphic versions to live. This gives two years for folks to adapt a much smaller set of code, replete with smoothly handleable warnings.
* By way of contrast, just reverting them to be monomorphic in 7.12 or 7.14 would break just as much code a year or two later. It is an option folks could advocate for, but it is an option that has no smooth upgrade path from the status quo, making it a very awkward option to enact at best. It has the benefit that the monomorphic versions of those combinators would have a place to live, but getting there requires much higher transitional pain in the absence of language extensions to make it possible. Now, one could envision a language pragma or the like that would enable us to go down this path. e.g. Lennart's WEAK pragma or by crafting a technical solution whereby you can export with restricted signatures. In the short term we took such technical fixes off the table as far too messy to address the issue, and when such proposals have come up in the past they've been fairly controversial.
So of those four options, I'd prefer to go down the third path in the absence of folks agreeing on extending the language in ways to make the fourth option viable, but there is definitely room for debate.
-Edward