The development of computer flight systems is characterised by lengthy certification processes, exceptionally rigorous testing regimes and a default position of extreme caution.
A change in any component at any layer in the hardware/software stack could introduce new incorrect behaviour. For example, microprocessors are so complex that different manufacturing revisions of the same model can introduce bugs.
As a result, these systems are very tightly specified at all levels and very slow to change, so that many systems still in production use what in most other contexts would be considered obsolete technology.
CPUs such as the 68040 and 80486 (both introduced around 1990) were designed into systems a quarter of a century ago that are still being produced today.
Summary of the problem
The pace of development of the electronics industry has accelerated hugely since the first generation of computerised flight control systems were introduced in the early 1970s, and is becoming increasingly incompatible with the needs of the aviation industry. It means that for example:
Devices now have manufacturing lifetimes of just a few years (compared with several times that a few decades ago).
Devices are now intended to have much shorter service lives than they used to (by orders of magnitude).
Increasingly tiny widths in microprocessors mean they succumb increasingly early to problems such as oxide breakdown.
Aerospace represents a very small portion of the manufacturers' business: their main customers make consumer devices, and have utterly different demands on the electronics industry from those of customers in aviation.
Research into the issue
These and other issues are outlined in some detail in Microelectronics Reliability: Physics-of-Failure Based Modeling and Lifetime Evaluation (and no doubt elsewhere - this was just one I found while trying to answer my own question).
The Aerospace Vehicle Systems Institute (AVSI) Consortium Project #17 Methods to Account for Accelerated Semiconductor Device Wear Out has attempted to quantify these risks.
What can the aerospace industry do - or what is it already doing - to mitigate the problems and risks this poses?