No 5 years is the max for unarmed violent assault, but then it becomes a grey area with attempted murder for the most serious category. That is life imprisonment potential (25 years).
The thing that stands out with Adam Johnson's case is not just his position, which the judge deemed to be one of responsibility, but the fact he lied in his evidence, didn't attempt to come forward, attempted to cover it up, and the constant grooming of a girl who had just turned 15, besides the sexual acts. He also comes out of it with the reputation as a sexual deviant, pretty much since his Man City days. Not that this is a crime, although the police did find some questionable porn. Apparently he's been treated for STIs several times, cheated on his partner with many others. Basically a sex addict.
They are probably making an example of him, but he's got the lowlife profile to go with it. And as I said before, anything child sex related in the UK has been super highlighted for a few years no.