It is not always the free upgraders that are eating up F inventory. I have a six segment trip coming up in paid F that is 3 weeks out and half of my flights are already almost full in F (less than 4 seats left), so UA does a good job of selling F seats on some routes.
UA tries to hold back 1-2 seats until the gate before giving those away for upgrades, but there are plenty of walk up F fare customers, customers on fully refundable F tickets changing their tickets, and some higher tier elite members on Y fare switching flights and taking the last seat.
I have a SFO-IAD itinerary at the end of the month that is on a Saturday, and F inventory for 3/4+ of the flights that day is already well into the single digits. There are a bunch of 757s flying this route, but it is selling. I can't imagine a weekday flight on that route and it is a no wonder GS and 1Ks are almost always in Y. LAX/SFO/SNA-EWR is another route that is almost always full in F/C with paid customers.