Legitimate question here, why does everybody seem to be obsessed with getting heavier duty vehicles (like the E400MMC and 6 cylinder StreetDeck) for routes that a lighter duty vehicle (like the 4 cylinder StreetDeck) would be more than adequate for?

Just because a route is currently run with B9s, it doesn't mean it needs to be replaced with a vehicle of equivalent power.
Take the 10s for instance, now granted I haven't used them for a while for obvious reasons, but I don't think there is a part of the route that a 4 Cylinder StreetDeck would struggle (I may be wrong on that though)
From my experience the StreetDecks used on the X1 seem to be perfectly fine.
And I think we can all agree that the B9s on the Cobalts are beyond overkill!