I've heard this a number of times, but how do you imagine this working?

For every legitimate case of a "diverse set of viewpoints" on some hot-button political issue, you have hundreds of crackpots and trolls who want to talk about free energy, telekinesis, chemtrails, and so on. Do you really want to have 50 versions of the article on gravity to choose from, most of them abject nonsense? Who gets to choose which one is given more prominence? If they're given equal weight, then the crackpots win the numbers game because there might be only 1-2 articles representing mainstream scientific thought versus dozens of "here's what I came up with in the shower".

I don't disagree that Wikipedia has some regrettable biases, but the solution probably isn't "allow all viewpoints". Look at the thread you're commenting on and the amount of whataboutism from single-issue accounts who seem to argue that the US is no different from murderous dictatorships.