So, an ergonomic reason?
some people have trouble pressing shift and alt together briefly?
How does that compare to the amount of injury, strain, and general issues people get from nonstop using the left mouse button? double left clicks are a lot of clicks, spread out across a grand total of 1 finger and 1 hand, added ontop (for the LCS people) of all the other left-clicks they’re doing.
These buttons are also used commonly, the user should have found a rest position that lets them press these for brief periods without strain.
You can even bring your hand down and to the left slightly and you won’t even have to move very far from a natural rested hand to press them, just bring a thumb in a bit. That’s hardly “ultra-flexible”.
The LMB workflow isn’t supposed to be wildly different from the RMB one. Workflow changes that happen there that aren’t related to the LCS system of doing things are highly likely to be implemented in the RCS keymap aswell. Hence keeping most of the 2.8 keymap intact as you say.