I don't think starting a new Blackberry, Symbian or J2ME project today is really a good idea, though. It takes at least a few years for a company to make an exit, and by the time you do, those burning platforms will have flamed out.
Whether or not that impacts your business model depends on what the business model is, of course. I'm sure a few more Symbian apps will be cranked out as retailers blow out their stock of old inventory. Very poor people will probably hold on to old phones for years, as well. But phones do break, and batteries die after a few years, so a decade from now Symbian will be just an obscure memory, like the Acorn Archimedes or the ZX Spectrum.
While devices might be affordable, the real problem is service.
In most countries you can get a cheap android indeed but having a data plan is really expensive or it's just not possible because of how plans are structured.
The iPhone was in part revolutionary because it came along with a $30 data plan as an obligatory add on. Before that you had to be in the most expensive cellphone plans to have access to data.
In the developing world you either don't have a data plan or you have a REALLY restricted plan and I mean like you get 300MB a month and if you go over you get hit with a huge bill.
Yes but let's not also forget that WhatsApp is 5 years old and 5 years ago it made sense writing for those platforms.
Not every app or startup is a messaging one and has the same goal of connecting people no matter what platform.
keeping as much of your codebase cross platform is also a good idea. which means java and/or c++