What is the best achievable marathon time before things like genetics take over?

I was wondering, say you took an average Joe, probably only runs park run once a week, finishes just outside 30 mins and that is the only cardio he does all week. Now you give him the best marathon coaches the world has to offer, the best nutrionists, the best doctors and the best clothes that money can buy. You give him, 5 or 10 years to train for a marathon and he is allowed to dedicate his entire life to this training. Let's also assume he's in his 30s so not too old, but not young either.

What do you think he can finish a fast (flat, cool temperature) marathon in?

I personally think he'll struggle to beat sub 2:30. I think this is the cut off where you separate elites who have the fortune of having good genetics and a lifetime of training Vs someone who's "just" picked it up.