I get the feeling this has been in the works for a while. Why else give Day the interim tag earlier this year when you have two coordinators with extensive HC experience? Pretty sure Urban has known this at least since August and picked Day as his successor.
This isn't an OU situation where the HC hung it up in May leaving no time to conduct a coaching search. If they wanted, tOSU has plenty of time to conduct a national search. This is a HUGE job. Deciding to skip the process and handing it right to Day tells me that Urban and the arhletic department feel fully confident that Day is their guy and that he can handle it.
That said, goodness those are some big shoes to fill. Simple common sense says we will take a step back from the last seven years. And we should be okay with that. Urban Meyer won at an obscene rate here and will be impossible to match. I just hope we can accept that.