I suppose that it is possible that this MGU-H design is so inherently superior to the design used by Ferrari and Renault that the power difference is impossible to overcome.
That said, I see nothing in the Sporting Regulations that prohibit teams from switching engines in mid-season. In fact, the FIA has required all PUs to have identical mounting points, presumably to insure that all PUS are similarly stressed, but it also opens the door for PU swapping. Of course, teams have contracts and Mercedes has limits on its ability to produce PUs. But, who knows whether a certain fizzy drink maker might be able to buy their way out of one engine contract and into another.
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