I think that the question itself presupposes some definition of "good," or "bad," which is not necessarily helpful and will just allow people to project their own preference on to the answer.
That said, I have previously given the issue of D&D's dominance a fair amount of thought. I would say that the following points are interesting, at least to me-
1. RPGs are always a niche hobby. Most people enter the hobby through playing D&D. Maybe the number of people who play D&D and then move on to other games is 10%. Maybe it's 35%. I don't know the figure. But the point is this- to the extent that D&D grows the pie, that means that there are more people out there that will eventually try other games. Which I think is a good thing.
2. Having a single game that is so popular does allow for a relatively thriving market for 3PPs. It is much harder to make money publishing products for games that don't have the reach of D&D. I would say that this is somewhat counteracted by the increase over time for people that desire to only play "WOTC" products, but even so, it is easier to make money on 3PPs for a game with a large market share than for a niche product.
3. I have previously stated that many people don't understand the lessons on D&D; that they have their own particular game that they like that they claim is designed better and that D&D is a poorly-designed game that just happens to be the market leader. I would argue that this misses the point; D&D is not a perfectly designed game for everyone, and, of course, it can always improve and get better (that's why we see it getting revise, get new editions, and have supplementary material). However, many of the aspects of D&D that some people don't like, whether it's the power-fantasy aspects (zero-to-hero), of the char-gen minigame, or the campaign, or the overly "game-y" aspects, or even the legacy of murderhobo combined with weak narrative ... those are the things that are broadly appealing to many people. In fact, the weak "prescriptive" elements of D&D (that you can play it so many different ways, even if isn't particularly great at any given mode of play) is what makes it so broadly appealing.
Understanding this crucial distinction is what can allow people to realize that D&D's design choices are those that are made to appeal to a broad undifferentiated market, and to move D&D to a more specific direction (for example, I think certain games, such as Ten Candles, are far better for melancholy horror, and I can usually create a bespoke game that will do a much better one-shot than D&D can ever provide) would actually be a bad thing for D&D. Once you understand this, you can realize that D&D's design is well-suited for its purpose, and avoid making pejorative and stupid comments about how people who like D&D for what it is are brain-damaged, or simpletons, or don't understand good design.