Are there any electronics whizz kids who can comment on this..
I wouldn't think of myself as an electronic whizz kid (I'm 32) but in the dim and distant past, I did my degree in electronics
Practical experience with cars has shown me that ECUs never fail all by themselves - there is always an external load or condition that causes their demise. For example, the engine ECU in a FIAT Punto Mk1 fails after the heater core leaks coolant. Or the airbag ECU in a Multipla becomes drowned when the windscreen leaks.
Sometimes the problem comes from a lack of maintenance - if spark plugs are not replaced, the larger electrode gap leads to a higher spark-over voltage, which means the low-tension side of the coil experiences a larger back-EMF, which might damage the coil driver in the ECU.
It seems, though, we have a third problem afflicting cars of the GTV's era - ECUs that we can no longer communicate with! I'm thinking here of the airbag ECU in my phase-1 that defied all attempts to talk to it. All I needed was to clear the 'pretensioner open circuit' fault code, but nothing I could find was able to connect and I ended up replacing the ECU instead. I don't even know the part number of what I replaced it with, but it dropped straight in and worked.
I'm not sure where you read about the lifespan of electronics being 15 years. Some components, such as capacitors, age quite badly, and the heat from continual operation of resistors may degrade their materials and the circuit board itself. But digital electronics tends to use fewer passive components (capacitors and resistors), so there is less wasted power, and greater reliability as a result.
I'd say the reliability of ECUs is the least of our problems compared to the poor quality of electrical connections and ageing of all rubber components on the car... particularly on cars imported from Singapore (a problem you are probably spared in the UK) where wiring insulation becomes brittle and falls off, I think due to ground-level ozone. A wiring loom with disintegrated insulation causes such maddening and widespread problems, it makes you want to use petrol and a match to sort the car out.