I think you underestimate how much specialists often earn. Let's pretend, just for argument's sake, that a GI doctor currently earns 400k/year (most are more than that, especially in PP). Let's even be generous and say that a PCP earns 300k. The extra 3 years of fellowship at, let's say 66k (to make the math easier) has the GI starting out 700k behind the PCP. With the income disparity, that only takes 7 years to catch up while the remaining 23 years of the standard physician career the GI doc just keeps pulling ahead - by 2.3 million when all is said and done.
I also think the uncertainty about reimbursement is somewhat overblown. Everyone talks about how radiology got hosed by CMS, and that's true to an extent. Most of them still make roughly double what primary care does. Is it less than the 800k they used to make? Sure. Is it still one of the top paying specialties? Yes, it is.