>Gmail is designed for everyone and has to support the full spectrum of users, in particular the new ones (because they're the ones picking new email accounts - consider that they're partially trying to target people who still don't have email).
If anything, the new Gmail with all the hiding and obfuscation going on is going affect the new users the most. The other users atleast know that something is possible and will hunt around for an option, but the new folks automatically assume that some things are just not possible. For example, editing subject when replying or forwarding.
You're assuming that most users want to edit the subject of ongoing threads (and thereby break the thread.) I have my doubts it even occurs to most them. So not having that option visible makes sense. I understand what you're saying about hidden options being unknown, but generally overall cognitive load is one of the biggest issues for new users and one of the best ways to mitigate that is to remove visual complexity.
If anything, the new Gmail with all the hiding and obfuscation going on is going affect the new users the most. The other users atleast know that something is possible and will hunt around for an option, but the new folks automatically assume that some things are just not possible. For example, editing subject when replying or forwarding.