Before the days of Paizo's adventure paths and the 5E megadventures I'd venture to say that most DMs I ran across had at least 3-5 modules they had bought. I can't say how many had run them start to end, but it did make for showing structure and giving prebuilt encounters you could drop into play.
Nowadays, I see them being bought exclusively to run and not for ideas or collecting. About 3/4 of the DMs I've run across who run 5E buy them (and usually just one before they start designing their own homebrew start to finish), less so for Pathfinder (the latter seem more apt to looking for free content online) - I'd say in the 50% range.
Seeing the numbers above, I'm not surprised though. The official adventures (beyond Yawning Portal) are a big ask - they're not cheap and you're basically slotting about 3-6 months of play to finish them. I've seen very few of the big adventures run to completion before petering out for one reason or another. Out of 7 such games that I know of for 5E, only one completed the entire adventure, for example. Doesn't exactly inspire folks to buy again if their first attempt falls through.