I’ve flown a lot between LGA/JFK and MIA this year and every time I have peeked at the upgrade list there have bern something like 20-30 people on it, usually with zero seats in F available (one time there were 2 seats available but that was an exception). Based upon this, the vast majority of people who have strived to get elite status in the hope of getting an upgrade will have been disappointed. I admit that this may not be representative of Delta as a whole, and possibly there are routes where upgrades tend to be more successful, but I suspect for these routes the extra cost of just paying for F in the first place would be quite modest.