First, RAMster works fine over builtin 1GbE and provides improvement over spinning rust, so 10GbE is not required. True, it's probably not faster than a local SSD, but that misses an important point: To use a local SSD, you must permanently allocate it (or part of it) for swap. So if you have a thousand machines, EACH machine must have a local SSD (or part of it) dedicated for swap. By optimizing RAM utilization across machines, the local SSD is not necessary... though some larger ones on some of the thousand machines might be.
The key point is statistics: Max of sums is almost always less (and often MUCH less) than sum of max. This is especially true when the value you are measuring (in this case "working set") varies widely. Whether you are counting RAM or SSD or both, if you can "overflow" to a remote resource when the local resource is insufficient, the total resource needed (RAM and/or SSD) is less (and often MUCH less). Looking at it the other way, you can always overprovision a system, but at some point your ROI is diminishing. E.g. why don't you always max out RAM on all of your machines? Because its not cost-effective. So why do you want to overprovision EVERY machine with an SSD?