It seems to me that once (if) you reach 100% crit, it would make more sense to swap out stormrazor for something else, like a Mortal Reminder/Lord Dominik's/GA, because the passive becomes irrelevant, but I never see anyone doing this, ever, including pro players. Am I just straight up wrong about this?
You're not wrong, once late game hits, Duskblade and Stormrazor both become a little more inferior to IE+Lord Dom/Mortal Reminder + 60-100 Crit. Duskblade and Stormrazor are only superior in the early and mid game where they have substantial levels of burst, but generally speaking, a lot of games don't usually last long enough these days to hit that kind of late game where you have so much extra gold that you can swap items out on a whim.
Well, if we get an 80 minutes world games where the 
are starting to spawn on fountain like some mushrooms people will probably do it. But yes, it becomes a very irrelevant item late game, only difference to a 60%+ crit build is the movement speed it grants on the first hit
thank you both for your thoughts :D
It doesn't lose ALL of it's usefulness. It does double the damage of the crit, also you do get movement speed. So if it is important for kiting in a situation, it might be useful still. I get what you're saying and agree to an extent.
If I were to hazard a guess, it's because attack speed and damage on an item is an exceedingly rare combination of stats and it gives a considerable amount of both.
To the point where with maybe a single crit item like PD, you could stack Stormrazors and still probably be pretty effective.
Not that you would, or should, but it's a good stat combo that's too rare in LoL.
On Sivir its not too bad of a late game item. Her w and the SR crit work pretty well together so there are some champs that should probably switch it out but others that shouldnt
