Importantly: they are repairable.
My Nexus S is a paperweight because Google kept it relatively closed. My BlackBerry Priv is even worse, since I can't root it.
Unfortunately, the reality with online devices is that:
a) They are sufficiently complex that they have an indeterminate (and effectively unending) supply of vulnerabilities
b) Effective vulnerability management requires vendor support
c) The market favors one-time ("sale") models, not ongoing ("subscription") models
As a result, vendors support devices for a fixed amount of time, often subject to arbitrary constraints (like going out of business or unexpectedly high costs of fixes).
From a consumer perspective, this usually manifests as computing devices getting worse over time--less performant, less secure, and surprising and arbitrary end-of-lifing.
You might argue that a more open device ecosystem allows open source contributors to fix vulnerabilities. This is true, to a degree, but only to a degree--just as (past some point) it isn't worth it for a vendor to keep patching a long-obsolete product with a tiny userbase, it's similarly true that an esoteric open source product won't attract a robust community that can meaningfully support it.
While free software advocates like to say "But at least it would be up to me to patch it," in practice, for most users (who aren't going to write their own patches, or hire a team of professionals to do so!), predictability, transparency, and fairness are the critical components in the products they use, not "openness" per se.
If phones had followed a similar open architecture as PCs then they might have enjoyed such support, as well.
i386 is kept as the arch name, but the baseline has went up.
Jessie needs at least a Pentium, and Stretch needs an i686.