I think the reason why I do worry about Jack's longevity in the game, is because his game, and his ability to be an effective forward is mostly based around his athleticism. As a key forward, the majority of his contested marking has been through his athletic abilities in the air, as compared to out-muscling his direct opponents. That's never been his strength. Jack isn't a monster key forward type who you can just plonk in the goal square and expect him to over-power his opponents. It is this reason I think guys like Hawkins will likely still be effective well into his 30s, as he is that type of player. Hawkins doesn't rely on his athletic ability in the air in order to be effective. With age, his strength won't decline, but Jack's ability keep being effective in the air will.
Yes he is a smart footballer who will still have a role to play next year and possibly beyond, but you can only go so far playing that 'decoy' role for Lynch, if opposing teams no longer see you as any threat as an effective forward yourself.
I'm happy to be proven wrong, but from what I've seen this year, it hasn't looked great for Jack. He has routinely been outmarked, and out-muscled by guys who wouldn't be fit to hold his water bottle in his prime a few years back.
If he is struggling mid-way through next year after a full (fingers crossed) pre-season, then there is cause for concern. He is still getting back into the swing of things after spending a lot of time on the treatment table this year.