They'd have their own design in 5-10 years.

Immune to shortages no. They're not suffering shortages because they don't have their own design, they suffer shortages because the whole supply chain has issues, starting from required minerals and going all the way to shipping.

And like the final product (commercial RAM) now goes to AI which pays better, processes/materials/factory utilization to make RAM would continue to go to another industry and not Apple, if that pays better then.