Yeah, but nobody knew the Steam version would exist when they bought this one. You're blaming the customer for ordering a cheaper sandwich with less meat because a new version of the same sandwich (that was not advertised at all) was gonna release in two months. (And even then, a sandwich is a one-time-use purchase while this game is gonna continually get better on Steam while itch.io gets shafted, so it's no perfect analogy.)
"nobody knew the Steam version would exist when they bought this one." and you are completely right! Not even Mike himself knew that his game would blow up and that he would have the chance to get one of his passion projects further than he would have ever imagined! so his cheaper sandwich gets discovered by subway and gets a polished up variant in their stores to buy just because it is that good! Mike saw the opportunity to develop and polish his game further. I would do the exact same thing.