The chart shows that Walmart revenue has grown 18% in 7 years, well below US GDP growth. Maybe they haven't dented revenue, but it's entirely possible they've dented revenue growth.

It also important to note that a corollary of this argument is that incumbents will still dominate their own sector - IBM still dominates mainframes. What matters to incumbents is whether or not those sectors are still relevant after disruption - mainframes less so, in-person grocery and shopping maybe more so.