1. It shouldn’t be the default behavior. Even if it’s suitable for “pros”, most Gmail users are not “pros”.
2. Even for “pros” it’s terrible behavior. Again, all it takes is one missed click and you are moving mail around, deleting it, etc. instead of typing a response. Why CTRL, ALT, etc. work fine to ensure that shortcuts are deliberate and not accidental for every other system but not Gmail is beyond me.
3. No one on this planet is actually writing email so fast & furiously that omitting a keyboard modifier like CTRL or ALT is going to impact their effectiveness. Even the fastest typers slow down to collect their thoughts, pause to consider their actions, etc. and no one is writing this much email in a day. I’ll go one step farther and say you could take keyboard shortcuts out of any given email system completely, and cost even the most-proficient, heavy user under 30 seconds in a workday. It’s a false efficiency. There are benefits to keyboard shortcuts for sure (vision impaired users and people with physical difficulties using a mousing device immediately come to mind), but “pros” are not a group I think is getting real benefit from it, even those who are convinced they are… and if they were, CTRL or ALT won’t hurt them, or any other group.

Bottom line is that there is no defending this UI/UX decision by any logic I can see. If there’s real evidence it’s better than having CTRL or ALT be needed, I’m open minded to it, but the benefits are beyond “marginal” and the risks are “high”.

J.Ja