He's a hell of a player but I don't think he warrants "2nd best RB in the league" money considering the years that Henry, Gibbs, etc. had. A 15M cap hit is plenty manageable in today's NFL -- hell, Philly damn near doubled Saquon's annual average salary but somehow lowered the upcoming cap hit -- but teams can't just shell out big contracts just because it's that person's turn at the table.
But if he gets 15M+, he gets it, and I'll say good for him. Shakir's emergence/steadiness made a notable difference for Buffalo this past season, but if we're talking numbers, the argument exists that Cook is the offense's second or third-most important player (behind Allen and maybe behind Dawkins).