...all economic downturns are labelled in hindsight. No one knew that the Great Depression was happening the day it got started. It would have taken months or years for people to start talking about it that way. That's how economics works. It's always retrospective in analysis, because talking about this specific present moment is incredibly difficult, and talking about the future borders on crystal-ball-gazing.The definition you cite for recession is one solely built upon hindsight. The market didn’t actually drop more than 20% from its Nov 2007 max until Nov 2008.
Actual news that a recession was occurring (not potentially going to occur) didn’t happen until last quarter of 2008.
Regardless of whether the thing began in 2007 (as the consensus of historians and economists agrees it did) or 2008, we can certainly agree that a clear and dramatic sign of Economic Problems had shown up before your cited date of September 2008: the collapse of Bear Stearns, one of the largest investment banks of the time. That occurred in March, and is where we get the phrase "too big to fail." The economy WAS in bad shape well before the June 2008 launch.
Edit for clarifying relevance: This means that we can, objectively, say that 4e launched during a really bad economic context, one that was unforeseeable by WotC even eight months before release. This is clearly an external factor which could (heavily!) influence a product's sales and popularity, even if it were universally agreed to be a good product. Obviously 4e was not universally agreed to be a good product (it would be insane to argue otherwise), but the point stands that there are and were major issues that happened unrelated to 4e's intrinsic characteristics that influenced its resulting success.
By comparison, 5e launched after the economy had definitively recovered (no timeline I have seen says the Great Recession lasted beyond 2013), employed much reduced staff the whole time so staffing was largely a non-issue until after publication (the whole "jury duty for one person delayed the conversion document by a year" thing), didn't try to make digital tools at all, and did not have to fight against 3rd edition partisans but rather had a lot of them actively championing it because they had finally grown weary of 3.5e after more than a decade of dealing with its issues and wanting (what they would call, though I would not call) a "moderate" fix. Combined with very intentional nostalgia bait marketing, a successful albeit not entirely accurate claim to "big tent edition" status, and a very popular (if, IMO, badly mismanaged) public playtest, on top of insanely expansive free advertising after release via Critical Role, Adventure Zone, and other D&D podcasts, 5e had nearly the best possible climate one could ask for to succeed. On top of this, it had radically lowered expectations because it wasn't trying to be a Hasbro core brand anymore, but a vehicle for coordinating with other brands (hence the MTG crossovers) and fostering other monetization (e.g. the upcoming film). Finally, by keeping staff low, outsourcing a significant portion of their work, and overall just publishing fewer books but specifically trying to make it so every book has some little piece in it that most customers will want even if they wouldn't normally buy the book for the rest of its contents, they have kept expenses very, very low while raising the per-book sales numbers.
These totally external factors have contributed to 5e being very successful in terms of sales. Again, this DOES NOT mean that 5e is awful and ONLY succeeding because of these external situational factors. But they are pretty clearly playing a significant part, and that makes it difficult, if not impossible, to conclude that 5e is a quality product solely on the basis of sales or popularity.