EzekielRaiden
Follower of the Way
Even 'dominant RPG' doesn't add up to 50 mil. IcV2 has the TTRPG market at 15 mil recently. Prior to the recession, industry insiders estimated it at 20 mil or so.
Which goes to show that, if there was any "failure" involved in 4e, it was a failure of WotC's expectations. They expected--or hoped--that they could revolutionize and revitalize the TTRPG market, in part by bringing it into the new millennium (technologically speaking), from more-or-less launch. That hope not only didn't happen when expected, it flat out never really happened at all. Refusing to do PDFs, failing to complete their online tools (particularly the in-house virtual tabletop), hardcore botching the presentation, and going for unwise (in hindsight and probably should've been foresight to boot) marketing strategies? All of those contributed to D&D being discontinued, without it needing to be a "failure."
Perhaps a different metric: Consider the Hulk and Superman movie franchises. You had Hulk in 2003, which established a backstory, then The Incredible Hulk in 2008, rebooting the franchise (as part of the new, interlinked Marvel movie franchise). But both movies earned approximately the same net income; if you subtract out the budgets, the two earned $108 million and $113 million respectively, nearly identical returns. Calling the first movie a "failure" because it was "replaced" only 5 years later seems rather strange, given the substantial success (over 75% return-on-investment--that is, regaining the full investment, plus at least an additional 75%--for both films).
Then consider Superman Returns and Man of Steel, a very similar situation. A gap of 7 years instead of 5, and earning slightly under 200% profit (that is, money earned *above* the investment cost) for the later film, as opposed to just under 100% profit for the former. It would be strictly wrong to call the former movie a "failure," yet its story and structure were abandoned in order to tell a new Superman story.
Now consider things like Harry Potter and other long-running book series. Is the line a "failure" because JK Rowling hasn't decided to write any more books of the same general type as the HP series? (I'm not really counting the Beedle the Bard book as an "HP-type" book even if it's set in the same universe.) I can't imagine someone actually thinking HP "failed" because it ended. Alternatively, you can have book series with a more "open-ended" nature--ones that aren't bound to individual plotlines or character arcs. Consider the relationship Sir Arthur Conan Doyle had with Sherlock Holmes. He came to hate writing for the character, so he chose to end the line. Does that make Sherlock Holmes a failure? If so, how do we explain the significant popularity he retained, such that Doyle could make profit off the stories when he started writing new ones again?
Now, of course, films and books and games are all different beasts, so the process and logic cannot map perfectly. But I hope this illustrates how something can be discontinued without being a "failure." By the measures of literally any other company (including Paizo, who freely recognize the runaway nature of their success), 4e is among the best-selling RPGs of all time--and certainly among the most talked about, for good or for ill ("...for there is only one thing in the world worse than being talked about, and that is not being talked about," after all.) If the line had "failed" by the standards of the industry, there probably wouldn't be a 5e at all; Hasbro likely would have shuttered the TTRPG branch of WotC entirely, or put them to work making actual board games (which are undergoing something of a renaissance right now, as I understand it), and sat on the D&D IP for a decade or two until it looked ripe for revival.
So, by that standard, we can see the existence of 5e as a sort of double-edged sword, with regard to 4e and its legacy. On the one hand, if 4e had truly "failed," I think it's unlikely that Hasbro would have bought the idea that a new edition could turn a profit. On the other hand, if 4e had met all expectations (which I do not consider synonymous with "success"--correlated, to be sure, but not synonymous), it's doubtful that anyone would have wanted to "try again" except to gild the lilly, as it were--and you'd think that would wait until the fire "died down," so to speak.
4e proved that TTRPGs could make substantial profit, but not enough profit. Calling "not enough profit" "failure" is going to leave a bunch of people skeptical. Calling it "success" is probably also going to leave a bunch of people skeptical. Framing it in simple black-and-white terms just leaves out way too much fundamentally important information.