If you want smart phone then you are limited to Android or iOS. Other options just do not count. iOS philosophy and devices which run it are not something I want to own/use so I am left with Android.
My first Android device was Nokia N900 with Froyo (Android 2.2) based NITdroid. When I saw “K9 mail” on it I knew that Maemo goes to trash (it’s mail client “Modest” worked only in landscape and used font size for visually impaired people). So few weeks later I bought Nexus S. Then Nexus 4. Next was Samsung Galaxy S4 which I won in some contest. Then moved to Nexus 5, LG G3, and now use ZTE Axon 7. Had/have few tablets as well: first some Tegra2 based one with Honeycomb (sold quickly), Archos G9, Nexus 7 (2012) and finally Lenovo S8.
For most of time I tried to run latest possible Android on my devices. Of course non-vendor one cause Android world cares about device for a year (or year and half in best case) and then ignores it. I stopped caring are there any updates to my devices. Sure, they are full of security holes etc but sorry I am not planning to spend few hundred euros every year to replace three phones and tablet.
With Android Oreo (not present for any of my devices) Google announced ‘Project Treble’ which should fix some of that. I suppose that in 2020 year 40-50% of new devices may support it. With old versions of Android anyway because binary blobs will be too old to keep up with newer releases.
Switching device is the other thing. Doing backups, restoring backups, (re)configuring applications etc. Last time I did factory reset on one of phones it took 2 hours before Google Play Store finished installing applications. Including those I removed half year earlier. Of course forget about text messages or call history. WTF Google?
Backups are fun anyway. Official way is “hope that Google keeps backups of your app settings in a cloud”. Most of apps to do sensible backup require root. Which usually require factory reset to be done first. Or all they do is provide other UI for ‘adb backup’ command (which does some backup and then decides to do nothing for any random amount of time).
ADB itself is a joke. Sure, it can be used to send files over USB connection but it looks like it’s authors live in 90s and all they have is USB 1.1 host controller in their PCs. I can not find other excuse for its speed of 3 MB/s (yes, THREE megabytes per second). Again: WTF?
My current plan is to use my Axon 7 with Nougat for about a year (or two) until it finally die or meet with ground one time too many. And still be pissed off any time related with backups (changing devices in family or sending them for repair).