I think Marner does not make the Hawks a Cup contender in the next four years, because that doesn't align with their window. Accordingly, the Hawks do not need a Mitch Marner type talent right now, and should instead just wait for when the team is looking like one that is a "Mitch Marner" away from contention (and you can determine if a "Mitch Marner" type player is the sort of piece you actually are lacking, or someone else to maximize your roster/cap construction optimally.
You're not even bidding on Marner today, you're bidding on Mitch Marner five years now. I'll have a much better sense of players five years from now, in five years (or four, or three..) than right now. Even if everything unfolds perfectly and you have a 33 year old Mitch Marner that is playing awesome and well worth his cap hit as a slew of Hawks draft picks are hitting their own primes and the team is ready to compete, I do not think acquiring a 33 year old free agent that fits the team well is particularly difficult, and you could likely get them at a term that would resemble the remainder of Marner's theoretical term.