Hmm, sealed as in completely impossible to open up for repairs? Sony should be able top open them, if necessary by destroying that leather patch and putting on a new one afterwards. I don't see why one would have to have a warranty to get a battery replaced, as long as you are willing to pay for the work and the parts. Apple offers such battery replacements for devices that are many years old. Watch companies will fix watches that are decades out of warranty, as long as you pay. The question is whether Sony offers this, and for how long - if this is yet another proprietary battery size, they may no longer be making them in five or ten years, and NOS batteries will have degraded just by aging.
Wel, usually some cottage industry arises that allows you to keep your old gear working, but that should not be necessary if the manufacturer did their job.