I can't imagine anyone replacing Myung, tbh.
I bet we all would have said the same thing about Portnoy before he left.
The thing with that is, I can think of loads of drummers who are better than Portnoy. Bassists of Myungs calibre are harder to come by, IMO.
The man warms up AND down, for crying out loud.
The problem is more like they are not into the music.
The greatest jazzbassists in the world could learn anything by DT and propably play it cleaner and better then JM.
There are a some people that propably would be interested though. Philip Bynoe, Jonas Reingold, Dick Lövgren, The bassist from Seventh Wonder (loved his string skipping technique)
Someone told me recently that Lars Norberg from Spiral Arkitect is the greatest technical progmetal bassist in the world.
Haven´t heard them actually but it might be true