I don’t see why it’d be impossible. Why?
First, because Framework does this on the laptop market successfully, and laptops, while of course bigger, also have some size and thickness restrictions.
Second, because smartphones tends to be bigger rather than smaller over time, and so there would always be room for the motherboard, particularly considering that batteries will become more and more dense in energy (we already have some smartphones that use silicon/carbon technology that offer greater energy density, and we are not even talking about semi-solid or solid state batteries yet).
Of course it requires to decide of a motherboard form factor and stick with it when designing new smartphones models so that it works both in new models and also in previous models cases. Which is exactly what framework does on laptops. Even though they improved the laptop shell over time, the motherboards are still compatible with all laptop shells they produced, because they put the motherboard form factor as a design requirement to cope with.
@WilcoBoode Why choose? Both are possible and in my opinion both should be done. Again, Framework is just doing that in the laptop market, you can change everything on a Framework laptop. Of course on a smartphone you’ll have things that would only be possible to upgrade via logic board change, as they are integrated in the SoC or soldered to the motherboard (and a smartphone is too small to not solder them) like storage, RAM, or network chip, but you still can have camera lens, microphone, speaker, … that could be upgradable (not just repairable), and even battery (when new more energy dense technologies emerge to get more capacities in the same battery size) without changing the whole motherboard.
In my opinion, a durable phone, and electronic device in general, should not only be fully and easily repairable, like Fairphone are (and of course Framework laptops too), but also easily upgradable (like what Framework is doing).
Also I think that is Fairphone decide to work and make full upgradability possible, they would have far more success than today, as whatever we think about other smartphone brands, most of the time even regular smartphones are durable enough to be kept for many year, even 5 to 7 years or more, particularly now that some manufacturers offer 7 years software upgrade support, and that independent repair centers has developed making repairs far more accessible even for regular smartphones (even when you need to change the screen, cameras or USB-C port).