Depending on the level of graphics needed, and the "intelligence" of the CRT box, you may well be correct. However, there were several old systems that successfully used CRT displays for a combination of low-rez digital gfx and real-time IR images. The A-6 Intruder was one of them.
Also, the current 747-400 can interchangeably use CRT and LCD displays in its instrument panel. I admit that the 744 does not yet display live IR or extremely high-rez synthetic vision-type gfx, but I am not sure that the CRT display alone would be the limiting factor in a future upgrade. That said, the LCD displays certainly are more reliable...
I also agree that from a Systems Engineering point of view, there is little reason for someone to try it, even as a retrofit. I doubt the market would be big enough to support such an effort, even if technically feasible.