Yeah, but it's not as bad as it sounds once you "move down a layer" and treat Tmux like your terminal manager rather than your terminal. When I need to a new shell, I don't open a new terminal window; instead I open a new Tmux "window" (the spiritual equivalent to a tab) and do the work there. I keep the same two terminal windows open with nested Tmux sessions for months at a time. By extension, opening new Tmux sessions is also an extremely rare event because the ones I already have are persistent.

> Might as well ship the terminal with tmux as a hard dependency and launch a tmux child process by default.

I think it's still nice to leave some room for customization here. Screen is still a pretty good Tmux alternative for example (in fact, five years ago you would have said that Tmux was a screen alternative rather than vice versa), and some people might prefer to use that instead.