Like Cookie Monster says, “C is for cookie and that’s good enough for me. But cookies are a sometimes food.” Treadmills are a sometimes workout. They’re not awesome for working on hills nor would I suggest doing your long training runs on them unless you have to. I mean, if it’s snowing outside or 4am or your stomach is flip-flopping or you’re feeling wonky or the wind is so strong that it’s crushing your will to live while running into it head-first for 15 miles, by all means, run on the treadmill.
Do what you gotta do.
Treadmills can make you faster if you can swallow your purist runner pride and see the benefits. Trust me, the treadmill can make you a better runner. Do you hit the ‘mill?
Now go out and run.