You have 2 round combats as 'typical'? Id like to see your encounter design and set up.
If using a longsword and as a human, then you get a lot less mileage out of the archetype for sure, just like if you have fewer than the recommended number of encounters per day. If you have a lot of encounters (and not much time to rest) then it comes back up again.
Things like flametounges, weapons that deal extra die damage, rogue or barbarian dips, or a regular source of advantage bring it back up again also. With an 18-20 crit range and advantage you have a 27.8 percent chance of every swing being an auto hit and a crit. Useful for particularly hard to hit targets at high level as well.
Assuming even just 3 round combats, 7 per long rest and 2.5 short rests at 20th level youre getting (4 x 28) or around 120 attacks per long rest (not including attacks of opportunity, polearm master bonus attacks, two weapon fighting, sentinel, GWM etc). The latter should net you another 20 or so attacks per long rest. Lets say 140 attacks per long rest at 20th level.
With an 18-20 crit range (and no advantage) you're spamming 2 extra crits every 20 swings. Unoptimised (human with greatsword) you're dealing 17 extra crits per long rest or 34d6 extra damage per long rest. Its around the same as the 21d12 you're spamming as a BM fighter (2 and a half short rests).
Grab a flametounge for an extra 4d6 fire damage on each of those 17 crits (and shove your target down on the first swing for a 30 percent crit chance per swing) if you want to highlight what a champion can do.