It depends what you mean by "flop" — yes, I know they mean commercial flop — but several of these have had great effects, even though money for the original companies wasn't among them:
Xerox Alto: I don't think I need to mention all the stuff developed in PARC that is now utterly ubiquitous.
Apple Lisa: A big influence on her more famous brother, the Macintosh.
Sinclair QL: Helped Linus Torvalds to decide to target Linux initially at the x86.
NeXT: Spawned OS X ; kept Steve Jobs in the computer industry until his return to Apple.
If IBM had retained DOS, I don't think it would have a 70% share. There were other PC operating systems that were somewhat portable, such as CP/M and whatever Digital Research was doing at the time.
My bet is that if there was no MS-DOS, one of the others would have risen in its place.
IBM's mistake wasn't allowing other PC operating systems to survive. (CP/M was only 8 bit and not a threat.) It was allowing Microsoft to license MS-DOS to other hardware manufacturers. They simply didn't understand the threat. They do now.
(Another way to think about it: If IBM had retained license to DOS, the only 2 laptops you could buy today would be Apple & Lenova.)
My first ever computer was a Timex Sinclair 1000. It was given to my by my uncle, who won it in a golf tournament. It came with a 16k memory expansion module, that plugged into the back of it, and a couple of cassette games, Frogger, and something else that I can't remember now. It took about 20-30 minutes to load the games by cassette. The membrane keyboard was terrible. Of course there was no modem or printer, so it was pretty limited. The best thing about it was that it was so small. Less than a year after I got it, the keyboard stopped accepting inputs, so I took it into the shop and smashed it to smithereens with a hammer.
Xerox Alto: I don't think I need to mention all the stuff developed in PARC that is now utterly ubiquitous.
Apple Lisa: A big influence on her more famous brother, the Macintosh.
Sinclair QL: Helped Linus Torvalds to decide to target Linux initially at the x86.
NeXT: Spawned OS X ; kept Steve Jobs in the computer industry until his return to Apple.