My previous phone was actually a Sony Ericsson W810i - pretty good phone back in the days. The battery of the phone was user-replaceable, but I never had the desire to do so. When it was replaced by the L720, it lasted around 1 week with little to moderate use. When it was new - in 2006 - I believe it would last around 1 week and a few days.
My point is: I don't think the L720 battery would need a replacement in the phone's lifetime. If it turns out that a replacement is needed, you can always remove the gorilla glass with a heat gun at around 70 degrees C. Then remove the screen and get to the battery. A new genuine battery costs about 25$ (it is relatively expensive compared to other Nokia batteries). You'd most likely also need a new gorilla glass with fresh/new adhesive. That costs about $30 and includes the digitizer.