"For example, in the old days we didn't have desktop environments. We only had window managers. So instead of being able to start Gnome or KDE from the system and receive a login screen, you'd login to your user account from the text terminal, run a script like "startx" that would have your preferred window manager and settings in it, and that would start the X Window System"
Uh... your id is lower enough to be around here by that time, but you don't seem to have been using Unix/Linux back then. XDM was first presented in 1988 and it was certainly part of the X Window System, nothing related to desktop managers (KDE is from 1996 and Gnome from 1997).
"when you wanted a GUI login, you had to run that as a separate app to replace the startx script, which made those use cases really klunky and error-prone."
Funny you say that. I only started to have problems with display managers (i.e.: remote session selection) when systems started to move away from XMD in favour of gdm and kdm.
"And not only was their no common sort of print dialog"
Yes, there was.
"Copy/paste usually only worked for apps that used the basic terminal paste capabilities; apps that had more advanced cut/paste capabilities were generally incompatible with each other"
Just like now. Apps always had access to the X Window buffer; it was non-well-behavioured apps those that didn't work (usually with roots coming out from Unix).
You made a seemingly cogent argument, only one that is not so much tied to historical facts.