Originally Posted by Bah Humbug

They have made a LOT more Di2 since that first iteration. I would expect the spare batteries to be readily available in retail channels for some time after production ceases, and if at that point you still have a Di2 system you intend on using for years to come, just grab a spare or two then.

If you want a system that will last 20 years... maybe stick to mechanical, though much of that will be getting replaced as well and may not be the same parts in 20 years.