Responses to the points made in the article:
1) I can’t easily delete my signatures.
Forget about deleting your sig and generally cleaning up email threads. Decent email clients do this on their own and are we really that worried about a few extra bytes of data these days?
2) I can’t easily change my email subject.
If its time to edit the subject of an email thread maybe it is time to hit the big read "compose" button and start from scratch anyway.
3) Ok, so what if you want to insert a link or something.
Drag and drop from the address bar. So easy.
4) Rapportive doesn’t work.
I've never used rapportive, it sounds awesome. Bummer on this one.
5) You can’t easily get access to people’s emails. (Also fonts and formatting)
This one does bug me, because they've all become 3 clicks (or new shortcut keys to learn). An option to just always display these things would be good.... but I've evolved and learned the hotkeys.
In conclusion, I've also been a long time gmail user who got cranky at the fact that the new compose form forced me to change my habits. I learned hotkeys and feel it has made me more efficient. Since I try to spend as little time in life being cranky and optimize efficiency I evolved and moved on.
Full list of hotkeys: https://support.google.com/mail/answer/6594?hl=en