Aside from the so-called "great horse manure crisis" story being somewhat vaguely sourced (horse manure was of value back then and was traded as such), even if

Aside from the so-called "great horse manure crisis" story being somewhat vaguely sourced (horse manure was of value back then and was traded as such), even if it were true or at least becoming a growing problem, I feel this problem is likely overstated. If cars had not removed it, we would've solved this problem by other means. Human "manure" was also as big a problem: sewers were relatively recent and still being build, and toilets were a brand new invention. Yet we've largely solved this issue without getting rid of people from cities.