There's a reason pace busses work as a regional thing, and don't have a route density high enough to quite match the city. Similarly, there was a trend for a few decades where suburban items who work in the city moved further and further, which stressed Metra. Longer distances, same customer count.
That said, historically, a large portion of South and west suburbia built up more, specifically around racial advances in history, from civil rights to economic participation to ending redlining. You see echoes of it in opposition to cta train expansions, for example.
That said, I'm sure you're right about some parts becoming reliably blue. That said, though, it's probably equally important to think about why. The Overton window has moved. Moderate republican policies in the 70s through 90s are "radical left" now, apparently. As a result, it crossed over some.
There's also something to be said for the absolute insanity we're witnessing. It's raving lunacy, literally, and they're fucking up the economy, too. The north suburbs in particular were always a bit more white collar, so I'd expect some awareness, there.
There's also some need to avoid assuming that democrats don't have bigoted ideas, at all. And if you combine everything, and look at the timing, there's a pretty clear "Trump effect", in which an increasing amount of lunacy drives people away. In the image, note that the first 7 is 2018.
It does beg the question, then, if we had a real moderate left party and a real moderate conservative party, would it stay that way? I'm inclined to imagine it probably wouldn't. So it's largely down to the abject extremism in the GOP. That creates potential to shift back if they calmed down.