It absolutely makes sense to work on a J2ME app today if you have a product aimed at developing nations. They won't be swimming in smartphones any time soon.
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.