Well, there's a difference between being "completely viable", "somewhat viable", and "never viable." Sivir and Corki are unviable because they fall in a category that's "strictly worse" than another ADC's, due to their kit overlap with other ADC's. Why play Sivir when Varus does a similar job of being an ADC-caster hybrid with lane dominance and AoE, yet does it better, for example.
Ezreal doesn't really fall into this because there isn't really an ADC that has the same advantages over him. Sure, some ADC's have escape and annoying poke, but none of them are as reliable or as spammable as Ezreal's. The question then becomes how large is this niche and is it universal. The most overpowered champions tend to have very wide niches, but that still means there's room for smaller niches, hence why gimmicks are even a thing in tournaments. Remember, jungle Ez into mass early game pushing was a thing during the NA LCS promotion tournament.