Not really that odd if you think about it - 707s aren't very well suited to the sort of duties that the 442s will be doing, and were on an comparatively high lease price, so ditching the 707s made sense, as did obtaining the 442s. Way back when the northern franchise was being let and awarded, 319s were the only option! We've only seen swathes of EMUs come off lease since all of the contracts were signed and the finances agreed. Electrification dropped because it is (unfortunately) far more expensive than anticipated, so we are back to the old DMU shortage situation we had before the swathes of electrification were planned and every little helps. That said, GW hasn't been competitively bid for for a while, and I would expect to see the sprinters moved on when they do next award a proper franchise.