A helpful thought experiment: what would happen to PS4 and X1X games if we cut the RAM in half?
Why, the same thing we saw at the beginning of the gen with PS4 vs X1 comparisons: lower resolution, missing textures, more pop-in, etc. And to remind the audience, PS4 and X1 had the same amount of RAM, one pool was just faster than the other.
Why would RAM help with those problems? Because assets on the disk and on the HDD cannot be loaded quickly enough. The CPU would issue an instruction to fetch an object on the HDD and would have to wait a few seconds before it arrived. Too slow.
So instead, the object is cached in RAM. Increase the RAM, increase the cache. Simple.
But what happens when the (slow) speed of the HDD and disk ceases to be an issue? Why, it would imply the CPU can issue an instruction to fetch an object and would not have to wait a few seconds before it arrived. Any caching processes related to RAM would also be quicker, reducing load times at the beginning of the level.
Not as many objects need to be cached in RAM. Increase the SSD speed, decrease the reliance on cache. Simple.
People who don't think SSDs will make a difference do not comprehend how gaming hardware works, full stop.