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I don't think you are a fool and I'm annoyed with DL for the cheap upgrades. I too fly paid F some of the time partially for medical reasons.
Last time I flew DL on a paid F ticket I was harrassed by a drunk guy sitting a row behind me and had to move. I've never had that experience on UA where the F cabin is mostly UDUs and paid F - people who typically fly alot and are not unused to the F cabin and how to handle the drink service. Mainly business travellers who treat everyone else as business travellers. The guy was treating the F cabin like a pickup bar and annoying me and the woman next to me he did not know when to quit and was too drunk to realize it. I complained to Delta and got a pat on the head nothing more. The FA moved me, informed the other FAs, and they stopped serving the guy but frankly they could have done more. I don't know that he got a cheap upgrade but he fit the profile of someone unused to the F cabin. He pretty much said so amid all the other comments.

Also I will add that if you were 120K miles/year on UA you'd be getting UDUs on domestic fights - the current FT stats are around 83% hit rate free upgrades and the SWUs etc are also pretty good depending on the route But nothing is guaranteed so you need to pay for F if you want a guaranteed seat unless you can confirm the upgrade ahead of time if there is e.g. NC space at time of booking, but this typically means using up one of your upgrade instruments. Since you fly in and out of ATL though DL should be your carrier of choice.

Here I am adding another paragraph! Another reason not to do the last minute upgrade - if your company works at all like mine - I can often book in F due to various rules but then if I try to submit a receipt for a paid upgrade at the last minute that would go through a different office and it would be hard to cover that. I.e. they will pay for F if I have a good reason for it but if I buy Y and then ask for the F upgrade cost later they may deny it. So it would be hard for me to do what you are suggesting, even though it might save money. They could argue that I booked in Y so I was obviously happy with that and thus did not need the upgrade.