So how many customers could this startup conceivably get? Unless you're a game with lots and lots of graphical assets, you have to be the kind of company with more ability to employ developers than to coordinate them (i.e. a unicorn).
It's also a moving target as limits are continually revised upwards. I assume they have done some diligence about their TAM, but I don't get understand the plan just yet.
Sure, everyone loves a rewrite, but perhaps Swift wasn't ready for it.
However, the cellular size limit is now 250 MB IIRC, so Facebook seems to stay just under that.
These older phones are way more common outside of the US and even just outside of the SFBA where not everyone buys a new $1000+ phone every 2 years.
Ignoring performance comes at a cost to you (server side resources) and your users (device resources). This is why generation after generation devices get more powerful and slower (from user point of view).
It is up to you to find out which point in the performance vs new feature/bugfix tradeoff gives you maximum value.