Not sure I follow your logic here.
Let's say there are two routes each of which requires seven four-car units in traffic, so the fleet of eight 769s could be dedicated to one of those routes with a maintenance spare. One route is 90% electric, the other is 50% electric, and we'll assume no difference in running time between the two types. Whichever way round they are allocated, there will be seven 769s and seven four-car DMUs in service. And the costs and emissions associated with diesel running are minimized by allocating the 769s to the route that is 90% electric, because they will spend less time running on diesel and the fleet of DMUs will be running on diesel whichever route they are on. This argument seems to me to hold for any proportions of electric running, until you get to a 100% electric route where it's pointless to use the 769s as EMUs can be used instead.
So what's the reason to put the 769s on a route with less electric mileage?