Sure, but you don’t necessarily need a SoC with an embedded modem. Fairphone could opt for other chips that do not have a modem, but are more friendly to free software (Allwinner, Rockchip, OMAP, …). The modem can be added as a separate chip.
Sure this will add some extra costs but it will be negligible considering Fairphone’s price range. This also comes with the advantage of physical isolation between main CPU and modem, allowing a better privacy control in case one needs to run a proprietary firmware on the modem chip.
Choosing the right hardware to avoid binary blobs not only comes with privacy advantages for your users, but also goes hand in hand with the longevity goal.
To this day I can still run the latest Linux kernel on really old hardware because I chose parts compatible with free software drivers. Had I opted for a different motherboard/sound card/etc. it would now be a piece of junk. No one except the manufacturer is able to maintain a binary blob, and they won’t maintain it for more than 6-12 months. On the other hand, with a free software driver, anyone can step in and port it to a new OS version.
Being able to run up-to-date software does not only mean you get the latest features, it also means you get the latest security patches. Sure Google still releases security patches for Android 5.1, but that won’t last forever. If you expect your Fairphone 2 to last 5 years then get used to the idea that it won’t get security patches long before that date.
All in all, Fairphone should not only strive to be fair in terms of production chain but also fair for its end users. Getting rid of binary blobs will go a long way in addressing this.