Ah, but there's the rub: you don't, and can't, get a guarantee of quality. UK4 is regarded as one of the very best adventures from the classic era. But for every UK4 or I6, you get a "Forest Oracle" or "Scourge of the Howling Horde". (Most adventures, of course, sit somewhere between the very best and the very worst!) And for that style of adventure to be worth WotC's while, they need them all to sell well enough - not just the cream of the crop.
Granted you couldn't be sure in ye olden days - some poor sods actually BOUGHT The Forest Oracle

, but with t'internet information is more freely available, opinions are easier to glean, and you can have a better idea of what you are buying.
Looking at my own 5e collection I have 4 hardback books - PHB/MM/DMG/SCAG - I've bought none of the big campaigns, and I probably won't. I also will not buy pdfs, I like to have something physical, colourful, with proper pages to turn - rather than a printed pdf loosely fastened with staples.
However the main beauty of the smaller old printed adventures is the ease with which they can be dropped into an ongoing campaign for something different - a change of pace. One of my 5E campaigns have just finished a lengthy homebrewed story, and have just begun a side quest - I've converted L2 The Assassin's Knot into 5E, made it suitable for 5th/6th level 5E PCs, adjusted the maps and deities, and it should be good for 3-4 good sessions of play. Not bad for a scuffed up 30 year old booklet which cost me £2 second hand - and preferable to buying a £20-£30 hardback adventure just to lift a chunk out which is suitable for my campaign.
I'll probably drop them into Castle Amber next...
And they have said they want to head north to their homeland to find out about 1 PC's orcish heritage, that goes through a big desert area on my map - aaaaaah I know, Master of the Desert Nomads and Temple of Death! And I'll make the Master a half orc - the PC's brother! And thus I get to DM my tattered old copy of this 2 module series for the 3rd time.
A lot of good stuff has been written about the Lost Mine of Phandelver - that nice introductory (shorter!) adventure supplied with the excellent starter set - if they can sell that, with rulebooks/box/dice for about £17 ($25?) then I'm pretty sure they can do smaller adventures for £8-£10. They can be condensed to 32 pages very easily - there's no need to have full monster stat blocks for every creature in the adventure, just refer the DMs to the Monster Manual! And if the text is kept more brief (less rambling scene setting and hand-holding) I would prefer that too.
But I'm of a different era, old, balding, somewhat nostalgic..... *sigh*