Computers have never used FPGA memory controllers that are upgradeable to newer DRAM standards so there's really no reason to say that's the "proper" solution. And memory standards overlap for 4-5 years which also happens to be the lifetime of a PC so the price/performance benefit of future-proofing really isn't there.
How would the increased cost of a FPGA memory controller benefit the end user?
They would still need a new motherboard to use newer memory types, because the modules will have different connectors. They might need a new chipset if the newer memory type wasn't actually addressable with the FPGA (not enough pins, not enough signalling capacity, not enough voltage flexibility, etc).
For the majority of users, they don't change the cpu, motherboard, or memory for the life of the computer (in many cases, some or all of these parts are soldered to the board). Paying more for flexibility that will never be used isn't good for anyone.
