The value of the upgrade really depends upon how much longer the OP intends to keep the 2015 S.
When we purchased our 2017 S, our plan was to keep that vehicle for 8 years - we're now halfway through our planned ownership, with 4 years left.
We decided to spend the $3000 for the MCU2 upgrade, new display and replacement radio module - because we'll benefit from having the new MCU for another 4 years.
But if our S had been a 2015, we probably would have passed on the upgrade, if we only had 2 years or less of ownership left.a
As for the benefits of the upgrade - the largest number of new features are only accessible while the vehicle is parked. If AP2+ vehicles are ever approved for FSD, those features could then be made available while the vehicle is in motion. But for AP1 vehicles that can never be approved for FSD, those "entertainment" features will always be restricted to when the vehicle is parked.
For us, the decision was based on bringing our 2017 S up to the same capabilities as our 2018 X, which now both have MCU2 and FSD.