Instant mail is a QOL feature.
Dual spec is a major game changing feature.
Those aren’t even remotely comparable, even calling dual spec “just a QOL feature” is just incorrect by definition of a QOL feature.
I’m in favor of some minimal changes to improve QOL or address bugs or exploits, but dual spec straight as it is from SoD or WotLK copy/pasted into Vanilla is absolutely not one of them.
It’s just dishonest to pretend just copy/pasting dual spec straight into Vanilla isn’t a major game changing feature.