The X Window System didn't immediately have X terminals

The tricky thing about justifying an X terminal is that it requires a nice graphics system and probably a nice cpu to drive that graphics system as well, so really the only thing you don't need is storage. basically it is hard to save money because you are buying most of a nice computer anyway.

wouldn’t you just need enough to render a window? i’m not sure if everything is sent pre-rendered or not

Think early 90's computers, and everything required to run a X server well. lots of memory, nice graphics, a nice cpu to move those graphics around. despite being technically thin clients, Dedicated X servers were not cheap.

It is sort of like the anecdote about an early sys-admin who traced down a problem with the new department laser printer locking up for hours to one engineer who had to be told to knock it off when he explained that he was printing nothing, But the printer had, by far, the most powerful CPU in the building so he ported all his simulation programs to postscript and was running them on the printer.