The maintenance burden for modern aircraft is in the upkeep of those modern systems. The mechanical systems are a lot more reliable, but there will already be microprocessors flying on the F-35 that are stockpiled by the supplier because they are obsolescent, or about to be obsolescent. At the same time, the price for "cracking the box" to modify software will start high and rise throughout the lifecycle. Despite processors being designed and installed with "X% spare capacity", the inevitability as systems and capabilities are added or modified is they will hit a processing ceiling and need to be replaced, with the associated integration and certification costs of complex software.
So just because there won't be a pool of leaking oil under each aging F-35, or a plethora of creaking systems with a 1 hour MTBF, don't assume anything got "cheaper". The costs just moved somewhere else.