Lack of popularity does not necessarily indicate lack of quality. Saying that 5E is a quality product does not diminish the quality of other products, nor am I saying it is "the best" or without flaw.
I may make the best whatchamacallit in the world, but it may not catch on. On the other hand if I make a thingamajig in an environment where there are many alternative thingamajigs and it sells really well and continues to sell better every year, I think that's a measure of it being a high quality thingamajig.
D&D was not always the best selling TTRPG and there's no magic associated to brand names. Many brands that were once popular have disappeared into the history books. On the other hand 5E has seen double digit growth every single year since it was released a decade ago.
My definition of quality is simple.
- Does it meet the company's goals? Yes. It's exceeded expectations.
- Is it reasonably consistent, free of errors, internally consistent. Again, yes. There has been relatively little errata.
- Does it meet the needs of the people seeking entertainment? That's the difficult one to assess.
This is a game of entertainment. We can't look at crash test ratings, we can't look at nutritional value, we can't do a scientific analysis. All we can do is find a scale to try to determine if people believe it has entertainment value over the long term. Do a significant portion of people who buy the game continue to play it despite alternatives? Does it grow? For 5E, the answer is yes.
With the rise of tools like Fantasy Grounds and Roll20, if you want to play any TTRPG published, you probably can. If you want to find a group to play an older version of D&D, you can do that as well. Yet D&D 5E dominates them as well.
Sometimes what can be a fallacy is not a fallacy. Appeal to authority is valid if you are accurately quoting a well researched and respected source. Appeal to popularity ... is not what I'm really attempting, but when it comes to entertainment I do believe looking at a games popularity, and more importantly it's continued growth is one of the best indicators of quality that we have. That doesn't mean it's "better" than any other game, that's impossible to judge and changes from one person to the next. It's certainly not perfect, nothing is.
I simply believe that we would not see a resurgence of D&D relative to other TTRPGS, we would not see 10 consecutive years of double digit growth, it would not dominate online tools, if people purchasing the product did not view it as a quality product. Is quality of a TTRPG subjective? Yes. That's why you have to look at the effect of many people's subjective opinion. That's not always a good idea, for an entertainment product I think it's a valid measurement.