Now, I'm a fan of specific, stylized flavors in fantasy rpgs. I've published a Gothic fantasy setting, a New Weird-inspired city setting, and a demonic apocalyptic desert setting. It's probably safe to say that I favor the high concept, bespoke, and special snowflake over the expected fantasy conventions and the well-trod ground of the high fantasy/sword & sorcery nexus. Even so, I want to tell you why vanilla fantasy is actually a good thing to have at the forefront of the hobby.
Vanilla Fantasy is the Lingua Franca of Fantasy RPGs
If you want to explain what D&D is like to someone who has never had the pleasure of playing before, it helps to have some established tropes and recognizable cultural references to fall back on. "It's like Lord of the Rings" goes a lot further than "Well, it's like ancient Tibet but everyone is a crab person and magic comes from stitching patches of demon skin to your body." The latter may be more evocative to the jaded palette, but the former is far more legible to a larger audience.
Vanilla is Translatable
Even if you aren't running a vanilla game, vanilla products still possess utility. Here's why: it's easier to add weird elements to a vanilla product than it is to strip away weirdness that doesn't fit the kind of game you want to play. "Weird" game products are often so heavily slanted toward a certain auteur-like conception of strangeness that they aren't even cross-compatible with other "weird" products. Vanilla products, on the other hand, can be more easily bent toward a variety of purposes and intents because they are made to inhabit the middle-ground.
When Everything is Weird, Nothing Is Weird
Rejecting everything that smacks of vanilla probably makes your game feel fresh and vital...for about three sessions, tops. Once the novelty wears off, all those weird elements will feel as rote and expected as elements derived from traditional fantasy. It's worth keeping in mind how vanilla works in the world of cooking: even when added in small amounts, it is an ingredient that helps bring out the punch of your other flavors. In most settings, the presence of vanilla setting elements makes the truly weird stuff stand out. Vanilla doesn't compete; it enhances the stronger flavors.
Vanilla Might Just Be What People Actually Want
Although you, in your rarefied DIY or indie circles, may deride vanilla fantasy, the larger rpg buying public might not share your views. In fact, sales figures seem to bear out the notion that vanilla sells better than more specialized kinds of fantasy. The reason the Forgotten Realms and Golarion are more popular than your favorite esoteric setting isn't because they have been foisted on rubes--it's because people find those settings comfortable and desirable when they're thinking about how they want to spend their hobby time. Not everyone wants to fight shit golems. Think of it this way: the more people who come into the hobby because they want to make a Drizzt clone means more people who will stick around long enough to delve into the wilder niches. Come for the Krynn, stay for the Krevborna.
They Make Vanilla So That We Don't Have To
I consider it a god-damn service that WotC and Paizo make vanilla fantasy because it means that I don't have to. They've got that arena covered, so I can make my own odder forms of fantasy. Vanilla and weird aren't in binary opposition; they're an aesthetic dialectic. One doesn't exist without the other. And since the other is covered, you have the freedom to go hard at the more idiosyncratic, personal end of the spectrum. But hey, if you want to try your hand at making a vanilla fantasy setting, more power to you. Frankly, going Full Vanilla might be the only truly radical and unexpected move left.