Marketing life on consumer products is on the order of six months before a there's a "new, improved" model. Avionics, including microprocessor based systems, must have a much longer life, both in durability, and in availability of replacement components. There are 50 year old Arinc avionics boxes still in revenue service, and 30+ year old boxes with microprocessors.
Avionics manufacturers will make last time buys for parts that will be obsoleted within the expected life of their unit: production life plus at least seven years. If their design uses state of the art electronics today, they will have to predict how many of these boxes they will sell into the far future, and the failure rate, then make a last time buy before the new box is even certified. This is just part of the reason for high prices of avionics.
The choice of Custom vs. COTS, Commercial Off The Shelf software, depends at least partially on the criticality of the application. Cat IIIc Autoland sensors and fright guidance systems get the greatest safety scrutiny, of course. The EFB, Electronic FlightBag, typically uses COTS, with some customized software.
GB