I would not call Essential phone a modular phone. The idea is to sell addons to the phone. The phone itself is the antithesis of modular. It cannot even be opened without destroying it. It got 1/10 repairability score from iFixit: https://www.ifixit.com/Teardown/Essential+Phone+Teardown/967... .
Man the handspring is an excellent PalmOS unit.
They were few and far between but I've got a few of the "cool" modules, a huge GPS, an SD card reader, a memory bank, and the "good technology" MP3 player.
Good actually pivoted from hardware to making an exchange compatible mail client for PalmOS to compete with Blackberry.
The slick Vizor Edge unit had a removable springboard slot, which triples the sub-cm thickness.
Handspring then made the Treo, which was competitive for what it was until WebOS and iPhone came along to spoil the party.
Palm Pre anyone? :(