There is extremely little indication that oil usage is the advantage for Mercedes, in fact it's Ferrari who were investigated hardest by the FIA, Ferrari who Horner and a couple of others indicated were doing it, Ferrari who brought an engine after the oil reduction limit and have been hit by immediate reliability issues and it's Ferrari who have been seen with various extra reservoirs around the engine that most people seem to believe are oil tanks.
The engines will need a massive massive redesign to work without the mgu-h as 60-70% of the electrical power won't be there, turbo lag will exist, the optimisation of fuel, exhaust temps, turbo speeds, heat, cooling, engine maps, literally everything would change even if the fuel flow rate was the same but it sounds like it's going to be 20% increased to make up for the lacking efficiency/lack of electrical output, they'll probably go from near enough 100% of time on throttle having the 160hp being available to more like 20-30%, or even less if the mgu-k output increases as has been suggested.
Worst part really is the unlinking of electrical output from the engine, going back to a kers push to pass is a HUGE step back in technology and one that doesn't fit in with any future plans for any engine company. You have two routes forward in the future, hybrids and full electric, that is why all the major manufacturers in WEC and F1 were happy with hybrids, because the R&D might be done for their motorsport divisions but it was R&D that would be done regardless, so in effect it's free/cheap for F1, it's just a reason to do it sooner, quicker and better under pressure of competing somewhere they can run the engines and advance the tech while the costs are high and long before they can get them in road cars. Going back to a basic turbo with a bolt on mgu-k is different, designing and making the best and most expensive possible race engines using technology that has no future is essentially just a waste of money that is only for F1. That is why the companies wanted hybrids in the first place because that is where their future was, Renault and Merc may have left/not rejoined in Merc's case I guess, if the 2013/4 engines weren't pushing towards hybrids.