In the past I have used a Neo Freerunner (with Om2007.2 and later SHR) and then a Nokia N900 (with Maemo 5). The former demanded plenty of your attention but could be made to work well enough, while the latter was a really nice yet exceptionally hackable consumer device. Before I switched to a Librem 5 I had carried a second phone with Android for about 3 years as the N900 was already too old for comfortable web browsing and there was nothing else on the market that I'd consider worth switching to for a while - but that was the only such exception.
I could probably go on and on. It feels like my little personal pocket computer rather than an appliance that someone lets me play with on their terms.
BTW. Funny how it can go from "it can't possibly replace Android/iOS" to "how is that different from Android/iOS?" :)