Not impractical if you're in the business of making money from the route, and a commonly made comparison. The question is better phrased in terms of what pax loading you've got and what size aircraft fits. A strong driver for the growth of the 777 (and the other twins) has been precisely because they are that little bit smaller than the 747 and can service a wider range of city-pairs at a decent frequency and good load factor. On the [few] very busy trunk routes of the world, the 747 wins hands down over the 777. Until the A380 comes along, and then the analysis of capacity vs number of aircraft needed will dictate the relative economics of the 747 and the A380. Back to the original question - the cost of ETOPS isn't a strong driver [apart from exceptional circumstances on a couple of routes].