"McDonald's" in plural

[Phaedrus]

Franzen is writing in a jocular tone, emphasized by the silly plural forms.

I agree with you about the author's tone being jocular and even sympathize with your finding the plural forms silly-looking and/or harsh on the eyes.

That said, imagine the sentence without those "silly plural forms." Revise it mentally so that Denny'ses becomes Denny's, Wendy'ses Wendy's, etc.:

". . . one eyeful of U.S. 36 north of Denver—the Denny's and Arby's and Wendy's, the tall man's cars in the big man's lanes . . . ."

Is it still clear that the author means to refer to more than one of each? Don't we need to add something, like "many" or "all," to make that clear?

". . . one eyeful of U.S. 36 north of Denver--the many Denny's and Arby's and Wendy's, the tall man's cars in the big man's lanes . . . ."
". . . one eyeful of U.S. 36 north of Denver--all the Denny's and Arby's and Wendy's, the tall man's cars in the big man's lanes . . . ."

I rest my case.