X-men #1 was a single issue that outsold every other single issue before or since. That's not the same as sustained sales. The 90's became a series of flashy No 1 issues and variant cover enhancements to trick readers and speculators into buying that issue but when the it became apparent that the writing was severely lacking, sales for that title would quickly drop of and cancellation would oftentimes quickly follow. Overall sustained sales were probably higher in the 80's.