If you're able to provide any assurances that this part will remain active and available for at least the next 15 years, that would be very much appreciated.
Thank you for contacting Intel community.
For your information Cyclone V SoC (5CSEMA6F31) is still active and available. Anyhow, the life cycle is depending on its order volume. We cannot assure the availability for the next 15 years but there is no plan for EOL in current time.
You can subscribe to our customer notification mailing list at below link so that you can be notified when an Altera(now Intel) device is going to be discontinued.
However, a lifecycle of a new device (eg: Cyclone 10) should be about 10-15 years depending on its ordering volume.