Really? So the way itch.io works is that you can't upload updates if you also have the game on Steam and/or GOG? If you say so.
Also I get that you like Steam. To some people all of those features are irrelevant. Also it's a bit disingenuous to imply that you need Steam to get mods. The only time you need Steam to get mods is if the mod maker refuses to upload it anywhere else... hmm, that sounds familiar.