I am not learning new Keyboard shortcuts only for Gmail.
Thunderbird (and other decent desktop packages) allow me to reuse the over a decade worth of training I have with my OS's KB shortcut semantics.
In addition, using the keyboard in a browser is a mess. It is very easy to not have the current context set to a different application, and Gmail's non-standard K/B shortcuts can lead to unpredictable (and destructive) changes in other applications (for example, accidentally entering random text in a document open in my text editor). Using standard OS K/B shortcuts means that if I am doing a search, and even if my current app is not the one I think it is, it's likely to still do a search, and not enter random characters.
That's a nice post hoc rationalization but in practice it doesn't actually come up. I don't have any other app with the same semantics as Gmail. For instance, what app combines forward/next navigation with selecting, labeling and archiving? Unlike vim which I miss in native text fields, there is no context outside of Gmail that the muscle memory wants to kick in; the corollary being that there is not a comprehensive set of OS default shortcuts that maps cleanly to the set of things I want to do in email. In fact Mailplane does attempt this for Gmail so you get ⌘-n, etc, but the common ones are really not sufficient to cover a power user's email workflow—you're going to need to learn a few things specifically for your client of choice regardless of what that is.
Thunderbird (and other decent desktop packages) allow me to reuse the over a decade worth of training I have with my OS's KB shortcut semantics.
In addition, using the keyboard in a browser is a mess. It is very easy to not have the current context set to a different application, and Gmail's non-standard K/B shortcuts can lead to unpredictable (and destructive) changes in other applications (for example, accidentally entering random text in a document open in my text editor). Using standard OS K/B shortcuts means that if I am doing a search, and even if my current app is not the one I think it is, it's likely to still do a search, and not enter random characters.