In addition to the design aspects, there's also supply chain aspects to worry about. Volvo could have long-term contracts with leather makers for x amount per year of leather for y number of years. Eliminating leather while contracts are still running mean that you'll be wasting leather or desperately trying to find a buyer until that contract runs out.

It's too late to guarantee the change for designs they've already started, and cars can take about 8 years to go from inception to release.

Does Volvo not charge a premium for an optional leather interior package?

How would that shorten the transition?

If leather is optional, then they are already building interiors without leather

Changing the amount of something has big effects on supply chain, logistics, and costs. Mass production is all about volume and lead time.

I don't believe the suppliers need a nine year heads up.

I didn't say that. If you buy less of something, it'll cost more per unit. Manufacturing line utilization and re-tooling are also important. Someone has to figure out how to change everything cost-effectively.

I find it hard to believe they need 9 years to that