I'm thinking Silver will be gone within a couple of years as a level in the program. In reality someone spending $3-6K with a $40Bn company isn't really making that much of an impact, and I'm speaking as a current FO. I was tempted to go for Gold, but that isn't worth a whole lot more these days. I buy the economy fare then buy up to comfort+ and the only difference to Domestic F is a narrower seat and 2 inches less legroom. That's a pretty good deal to me.
Though I've been somewhat surprised this year to get 2 upgrades, one on HNL-SLC and the other from PHX-SLC though the latter is probably due to the fact I only bought a ticket 4 hours before departure so that probably bumped me up the list a bit.