Are you trolling? The environmental impact of reduced traffic is far more significant- and currently observable during this quarantine- than a hypothetical horde of hikers.
Cities centralize services and make them more efficient. Groceries, trash, hospitals, etc. By spreading all of these things out you make them less efficient. It seems you're overlooking everything else and focusing strictly on commuting's negative effects.
Perhaps, but those images of clouds of smog over LA or Beijing- though admittedly they might be largely produced industrial activity rather than transportation- certainly elicit a visceral response.