It's also quite common that OEMs hold spare parts available for around 25-30 years. Which partly means the suppliers of these parts and also the µC manufacturer

It's also quite common that OEMs hold spare parts available for around 25-30 years. Which partly means the suppliers of these parts and also the µC manufacturers have to held spare parts available for a long timeframe. However these spare parts usually don't come with updated or even maintained software. The lifecycle for SW currently is: Car is developed (3-4 years), then car enters the market and some minor updates/bugfixes are added (probably also for 3 years) and then nobody touches anything anymore. That worked out reasonably well for the last gen. However things will get very interesting for the current and next generation of vehicles with lots of connected features - and also driver assistance features which rely on connectivity. If the same model is applied here unlucky customers could be unable to use lots of features after only a fraction of the cars total lifetime. And of course they could be driving around with security holes.