What kind of development would we be talking about? I still use my 2009 MBP for day to day development. It's ~10% slower than a Core M3, I usually do webdev on it, but it's also fine for AOT things like Rust, Haskell and even some C here and there (I recently compiled RethinkDB, no biggy).
Things that really hurt are short battery life which is ~1.45hrs, and that it misses the modern CPU extensions which will obsolete it soon. Both of this issues obviously won't be present in a Core M3, so I see no reason why you wouldn't dev on an M3 unless you compile big native projects with great frequency.
I'm surprised you find Rust and Haskell bearable. Because for both languages slow compilation is a pretty well known problem[1][2]. On my old laptop (2011 MBP) compiling and interpreting (ghci/ghcid) took so long that it was distracting. This isn't even a big project; just a medium-sized one with a little over a hundred Haskell modules.
Well to be fair I only do hobby projects in both languages. My Haskell project is a C compiler, and my Rust project is a game engine, both compile in seconds. Both have just a few modules. Most complexity is in the dependencies, which have some compile time consequences but not terrible so far.
Things that really hurt are short battery life which is ~1.45hrs, and that it misses the modern CPU extensions which will obsolete it soon. Both of this issues obviously won't be present in a Core M3, so I see no reason why you wouldn't dev on an M3 unless you compile big native projects with great frequency.