Use of tristates vs multiplexers in a RAM
It's an issue of "fan-out" or "fan-in". So your theory is more or less accurate. However there is the additional benefit of using tristate outputs that you can attach them to a bus! If you use a CMOS Multiplexer you cannot share the wires that the output is on, if you use Tri-State devices then you can share the bus with other bussed devices (under the control of an arbiter for example)... in the context of RAM, think "memory bus."