I see I was correct in guessing that you weren’t dealing with Linux, since that’s almost certain to use GNU userland, and there tee is part of coreutils, and good luck getting anything done without coreutils. (GNU sed is its own thing rather than being part of coreutils or any other *utils bundle. I don’t know why this is.)

I’m still a little surprised to hear of tee not being present when sed is, but I’m not familiar with BSD. Maybe they care even less about POSIX than I would have expected?

I’ve been a little bit embittered against sed because of the differences across platforms. As usual, the version that comes out of the box on macOS is ancient, for licensing reasons, and the distinction has caused me grief too many times. I seem to recall having to give up on using sed for in-place regular expression replacement in a file when I was only caring about Linux, Linux under WSL 1 and macOS, and ended up using a fairly convoluted Perl one-liner instead (something like -plni.bak followed by rm foo.bak, because -i sans extension broke under WSL 1). Cross-platform operation is trickier than I wish it were!

I prefer ed for so-called "in-place" editing, not "sed -i". I am always using tmpfs anyway and it makes little difference to me if I can see the temporary file created or whether it is effectively hidden. Even NetBSD has added "-i" to sed now, and coreutils does not include ed. I do not see such moves as "progress" but that's just me.

If MacOS sed has "-a" then something like

   sed -na 's/a/b/;H;$!d;g;w'file file

should approximate "-i" (but without a temp file). The catch is it will leave a blank line at the top.