I'm all for adding native ACME support to web servers, but I can understand that they'd rather wait till ACME reaches RFC status (which hopefully won't be long now - the draft is in WG Last-Call).
Once that's done, I'd definitely be disappointed if web servers still decide it's not in scope for their core product.
I don't think it's unreasonable to expect an officially supported optional module, at the very least. It's how they've handled experimental features in the past.
Once that's done, I'd definitely be disappointed if web servers still decide it's not in scope for their core product.