Doesn’t it depend on how competitive the other 20% is? Having one player with 80% and the remaining 20% split amongst 20 companies would indicate market dominance and that the others are competing with each other and not meaningfully with the market leader. I think the Herfindahl-Hirschman Index[0] is used in these cases but I am not sure what it looks like for search.

[0] https://en.m.wikipedia.org/wiki/Herfindahl–Hirschman_Index