It's a bit of a strange situation because when he's healthy in the playoffs, he's an absolute beast. However, he's been injured at some point in each of the last three playoff years, which has diminished his effectiveness. Is injury a concern going forward? As he ages, the injuries will only increase in frequency. It's definitely something to take into account when negotiating his contract.
The quality of linemates definitely explains his relatively weak 5v5 numbers away from McDavid. However, if he's asking for $14 million, he should be able to make superstars out of the McLeods and Fogeles of the world. That's more than Matthews and MacKinnon, who, IMO, are better players and signed their contracts at younger ages. Add in the injury concern mentioned above and I think $12-12.5 is more reasonable.