I think 'walled garden' is the perfect description. There are promises of all the wonders within, but you can't find out whats really there without paying the price of admission (sign up / install their software platform / whatever). You can't peer in from outside (anonymous viewing), and you can't check a catalogue (i.e. the contents are not indexed by outside search engine as far as I can tell). And as such, if/when the head groundskeeper decides to change the price of admission, or demolish the whole place and build a carpark, any content is likely lost.

Yes, I'm old and grumpy. Sorry.

As robthebold said, VoLTE seems the biggest single risk to SFOS future. My biggest annoyance with SFOS is that after 7(?) years and 5 handsets, I've still never acheived a reliably working telephone. I get random call drops, lost audio, randomly forgotten or ignored bluetooth headsets. There are other issues, but over the last year, working from home more than ever, has made me realise how annoying it really is. Working for myself, there's only so many dropped or missed customer calls I can take before it stops being funny.
I have seriously considered a dumb 4g phone several times, but I don't think I could stand sending SMS that way in the 21st century