Check out Chrome build time benchmarks between 3/4xxx and 5xxx series (or any dev tool related benchmarks). 10-20% off your iteration cycle is noticeable - and that's a single generation jump.
Increasing iteration speed is a big productivity boosts, seeing professionals with 5+ year old devices is just ridiculous to me, you spend 8+ hours on that device - investing 2000$ for an upgrade every two years is not that much to ask.
Compiling Chrome is a nice benchmark but in reality dev cycles are quite smaller (or you might want to optimize your toolchain).
Finally, it's quite common for developers to forget the kind of hardware their users are running on. Then they produce software that only runs well on the next gen hardware. If you develop on a beast, then at least test whatever you are producing on a 3-4 year old machine to see how it will work for the clients.
I have a 2014 retina iMac. At some point in the next few years, I’ll probably replace it with an M series iMac, and then put Linux on it. It’s hard to imagine that it will ever really reach end of life due to speed at that point.
edit: form factor, performance, build quality.. is also up there, I just didn't mention it, because I think the battery life is the killer feature of the M1.