And why does the last Handheld from Sony have to last longer?
Actually Sony could stop producing Vita within the next few month as well as in a few years.
It's cheap right now because it uses components that are still standard, though slowly are getting outdated. A complete refresh might be to expensive at one point because of low sales. Producing outdated hardware gets expensive at some point as well though.
Games for Vita will get less at some point as well.
So there's actually zero guarentee that Vita will last longer. No guarantee that it lasts shorter as well.
That will depend on costs and incomes.