All valid considerations. I would also like to reach 100% crit, but the opportunity cost is quite simply too high.

So first thing to know is that the chance of missing two crits is actually much lower than 1/25, because League uses pseudorandom chance, meaning that it will trend toward ensuring both shots crit by some high amount. It was reliable enough that, for any purposes I found during testing, I always had at least one crit per two shots.

Critical chance runes are also extremely inefficient; to get 20% from runes would require literally making every mark, seal, glyph, and quint a critical strike rune. That would be a pretty big hit to survivability compared to the runes being used now, which overall would lead to more gold being generated (through a stronger laning and teamfight phase) and overall more damage (through surviving longer in fights)

But on the cost-benefit side, it's heavily suggested to get Lord Dominiks around the upcoming late game, far more than crit chance, since this build suffers more heavily against tanks compared to the standard build. Crit Chance does more poorly by comparison.

That would still leave one slot, but going for Statikk Shiv or Phantom Dancer as a sixth item is going to feel really bad because they add significantly less attack damage compared to Youmuus or Duskblade.

It really all boils down to opportunity cost. You can't get 20% more crit chance for free; something has to give. All the options for another 20% require great sacrifices either to survivability (runes) or damage (sixth item). The only reason to get it is to ease the mind of an RNG screwup, but it's already a statistical guarantee under a pseudorandom system anyway, which means raw damage or survivability is the name of the game by the time that 20% becomes relevant.