Note that a single postgres instance with one core and 1gb of ram on 1 ssd can “scale linearly” by my definition, since you can easily double, triple, etc all those specs.
On the other hand, a fully populated data center running scale out software at 100% power capacity can’t scale linearly anymore, because you can’t upgrade it anymore. For that data center, power or maybe space is the bottleneck.
Short answer to the question you asked: SSD’s are not going to be the bottleneck for software that’s migrating off disk, and the database probably won’t either. Hardware trends mean the storage and database just got 10-100x faster, while the business logic maybe doubled in speed.
If the system was well balanced before, that means no one spent then-unnecessary effort on now-necessary optimizations on the compute side.