I've switched from screen to tmux about a year or two ago, after decades of using screen.. and I'm perfectly happy with tmux, as it fits my needs.
However, it's not true that screen has no advantages over tmux. Each program has its own strengths and weaknesses. For instance, screen is scriptable via its Lua bindings, tmux is not. Screen has zmodem support built in, tmux does not. There are probably many other examples, since screen has a bazillion features which have been developed over decades, while tmux is relatively new (and its developers don't seem to care to duplicate every one of screen's features).
tmux is scriptable from the shell via its command-line arguments, so any language can script it.
It's certainly true that screen does have tons of small old features... but I sort of swept that in with "unless you're using HP-UX or something." zmodem support isn't exactly relevant to the overwhelming majority of programmers, but yes, I admit these things are still relevant to a few niches.
"tmux is scriptable from the shell via its command-line arguments, so any language can script it."
I'm not sure if tmux is scriptable to the same extent that screen is via screen's Lua bindings, as the latter probably exposes many of screen's internals, while scriptability of tmux is limited to what you could do with its command line arguments. I haven't researched the matter, though, so I may well be wrong.
However, it's not true that screen has no advantages over tmux. Each program has its own strengths and weaknesses. For instance, screen is scriptable via its Lua bindings, tmux is not. Screen has zmodem support built in, tmux does not. There are probably many other examples, since screen has a bazillion features which have been developed over decades, while tmux is relatively new (and its developers don't seem to care to duplicate every one of screen's features).