Sure users want the limitations lifted, but most of them don't understand that some of the limitations are inherent, and generalized existentials won't eliminate the pain.
I loathe writing type-erasing wrappers as much as the next guy. But generalized existentials don't eliminate the need to write them, nor (AFAICS) do they make it much easier to do so. An existential Collection type would not remotely replace AnyCollection, because that type would not conform to Collection. In fact, most basic things we expect from a Collection would not be available: you can't index it, and first() could at best return Any?.
Nobody has proposed a set of features really targeted at the pain points you cite; that would be a really interesting exploration (I suspect the answer has at least as much to do with solving what I call “the API forwarding problem” as it does with existentials). Even if what is being proposed makes incremental progress toward solving those problems, though, I fear that in the specific place we arrive, having applied that increment, we'll have done more harm than good as explained in my opening message.
I totally understand we have problems with the limitations on existentials, but before we charge ahead lifting limitations IMO we have to consider how the whole language works and fits together once we've done that. The future I see when I do that mental exercise has what I consider some serious new problems—problems that aren't addressed by simply saying “the limitations are bad so we should lift them.”