My thinking on mobile devices, and voice/comms more generally, is that we're probably better off separating the telco-link, voice, and general compute capabilities.
One element of this could be a WiFi-only phone, effectively a VOIP endpoint which relies on whatever local WiFi is available for connectivity. Where a fixed-point WiFi isn't present, that phone could rely on a cellular WiFi hotspot. Given the various vulnerabilities and proprietary aspects of cellular comms, this at least offloads the security concerns to its own device.
Increasingly people are relying on Bluetooth devices to hear and speak through smartphones. The logical extension to me is to move the primary phone guts entirely into a Bluetooth earpiece or headset. This would handle the voice aspect of the call itself.
Palmtop computing appears all but entirely dead (with some DIY/hobbyist exceptions, e.g., the MNT Reform Next (https://www.crowdsupply.com/mnt/mnt-reform-next>). There are also small-form-factor laptops, with 12" and smaller screens being comparable with tablets (and even some of the larger monster smartphones available today), but offering a full, user-controlled, operating system.
There's still the problem of software availability in an iOS/Android App-dominated world. It's technically possible to run many of these within a VM or emulated environment, but some app providers, particularly for financial interactions, attempt to detect and disable this. Ultimately regulation and/or lawsuits may be necessary.
As power-hungry as they are, mobile devices do make remarkable use of available battery capabilities, which is a place traditional laptops will struggle with. There's the question of having to carry multiple devices, though I suspect many of us are doing so already. There's also the possibility that market segmentation will make such users attractive to business and other institutions, reducing the barriers that presently exist.
All that said, I do applaud the FSF's effort, though I would like to see other paths (such as the one I suggest here) pursued as well.