[r00fus]

It's a battery of a popular but specific chemistry in a specific shape. Given this is Apple, those may be popular chemistry/shape, but if not, then in 10-15 years it's not going to be easy to find if Apple doesn't produce them. I'd say Apple keeps the supply chain around for maybe 5-10 years.

[turtlebits]

Apple still does battery service on iPods, as early as the 2009 iPod classic.