A very strong former pro (owns a local shop) was talking about the old days of 53x42, where the absolute smallest ring you could get for those cranks (144 bcd) was a 41T. For flatter races he'd pair it with a 12-18 (7 spd), and for rolling terrain a 12-21, and hilly would mean going to a (gasp!) 23T. We're talking 3 mile long slogs. He laughed at himself, saying something like, "I have no idea how we did it."
A fellow competitor of his, a former Cat 1 and former state champion (120 mile road race, attacked at the first climb on the first of 5 laps, took that former pro and 2 others with them, and they all stayed away to the finish, course had steep climbs and long climbs), said something almost identical. He runs a 39x25 as a low gear and sometimes thinks it's not low enough.
It took me a while to get to that point, probably 2001? when I got a Campy 9s group (135 bcd so 39T was smallest, not like Shimano/SRAM's standard where you can run a 38T). Until then I ran Campy cranks that had a 53x42.
Another former pro, former pro team director, told me a few years ago that his ideal setup is a 52/36, and that's what he ran on his bike. He's a strong road racer, fast, and he was good with that and an 11-23 or 11-25.
One thing I realized recently is that if you're going a certain speed up a hill, say 8 mph, you're doing the same work no matter what gear you're in. A big gear may make you think you're working harder but you're not - it just feels that way. Spinning a small gear at the same speed is the same wattage but you don't build up lactic acid as much because you're taxing your aerobic system first, muscles second. Pushing a big gear is sort of anaerobic, depending on how slow you're pedaling.
I blow up either way but it's more comfortable blowing up while spinning than while pushing.
Right now I happen to be running a 55/44 setup with 11-25 for training and 11-23 for races (flat terrain mainly for all riding). I'll be going back to the 53/39 as soon as I overcome my "don't feel like working on the bike" inertia.