Where they have put it is a lot more useful for cars that then drop south to the 401 ... I assume they'll upgrade the 35/115 between there and the newish expressway portion of 115 just north of their, and eliminate the only at-grade interchange.
And although I make fun of those worrying about highway numbers ... let's hope they simply renumber 115 as 7.
It would still need to be 407, because both 7 and 7A continue west of Peterborough. It would be 407 until where Highway 35/115 currently ends in Peterborough, and then becomes Highway 7 (which would make sense from a natural transition persective, Highway 407 actually becomes 7). Only problem is what would you call the small section between the 401 and the 407? 435?