There wont ever be a case where RAM will be replaced by SSDs or any other memory type,
Its like saying lets replace the L1 cache with Ram, exact same idea, the problem is the future development of computing technologies wont permit such a mad idea, L1 cache operates at lets say 30GB/s , and assume ram can only hit 12 GB/s , on the same scale SSD's operate at 500 MB/s ... So we can see SSD's are climbing, but if you can develop chips for SSD's that will hit the desired 12GB/s to replace Ram then there will already be a successor to Ram that will be able to hit even higher data rates, and most likely there will be even faster more efficient L1 cache, and they will all be built on advances of technology that are all closely related.
The difference ultimately resolves down to proximity, L1 cache can hit phenomenal data rates and minimal latency because of its proximity to the processor, i.e. on Die, Ram has a dedicated memory controller that gives it a direct pathway to the CPU thus lowering its latency and upping its data-rate. SSD's on the PCI Bus are a level of abstraction further away than this, thats not to say the Innards of SSD's aren't helping the advancement of memory devices and controllers the world over, but they are not fit for the purpose of bleeding edge speeds provided by RAM , L1 and L2 caches, And they will never just be conglomerated into a single concept,
There are tradeoffs everywhere, The 3 core concepts are super-fast cache for continuous processing (feed it in --> calculate --> feed it out ) in 5 cycles, Getting data from Ram (to continually feed the cache) is somewhere between a 50-200 cycle operation, And from hard drives/ ssds (whose purpose is permanent storeage of mass amounts of data) on the PCI bus its even more costly, albeit better with SSDs . But its still all about tradeoffs, that speed comes at the cost of high TDP in Cache, and a complex memory interface for Ram, growing the interface to accommodate SSD scale amounts of GB's at a data rate comparable to RAM and with just as low a latency is something that cannot be handled with the current tech, And since we are talking about future tech i say my Point still stands, the 3 core concepts are linked , but distinct. RAM will still be RAM, permanent storage will still be slower , even as the technologies grow in speed the divide will Always exist.
