Firstly, RAM is still significantly faster than both your regular 6gb/s SATA or even the newer PCI-e based solutions. RAM is also designed to be written and erased repeatedly, at the cost of volatility. RAM generally doesn't wear out due to regular use - though, of course, it can fail like any component.
While the lifespans of SSDs have gotten much better, SSDs do wear out. They're absolutely brilliant for nonvolatile use, but if you wrote and overwrote NAND (which SSDs contain) like you do RAM, it would wear out.
Both are really optimised for different things, and you're better off having enough RAM (and using SSDs or spinny hard drives for paging out) than compromising on enough RAM for the task.