I know the conversation has moved past this somewhat, but it's strange to see people think Master Chief would be even close to reviled as an inclusion by more than a very small sliver of fans (though I know it's mostly only one guy saying this).
Speaking just in terms of the west, getting Master Chief would be tantamount to getting Cloud, or, given the history, a mix of Cloud and Sonic. Except, as opposed to Cloud, now people have come to terms and embraced getting characters that don't have strong connections to Nintendo, so it'd be like getting Cloud without the half that found it against what Smash was about.
Suggesting otherwise seems to indicate some ignorance as to the pulse of the industry. Or at the very least, an unawareness of the esteem of Master Chief, even among those who aren't super familiar with Halo. In my opinion, which I don't think is an uneducated one, I would position Master Chief as the biggest gaming character not yet in Smash. At least outside Japan. And lest we forget, that while a crucial region, Japan is only one country.
To that end, I'm sure you could raise anecdotes of people who would be less than thrilled with his inclusion. Every inclusion will elicit groans from someone. People who would actively dump Smash because of it? Well, that seems a little extreme, but maybe it exists. The thing is, I'm sure for everyone who swears off the game because some optional character bothers them, Master Chief would bring such a tidal wave, such an influx of eyes (and with it, units moved), that the former contingent would be just some afterthought, really. A drop in the ocean.
I mean, from my own anecdotal experience, I remember when Halo was at its peak - during the original trilogy. I remember it very well; those games were always being played at my house, the houses of boyfriends of the time, the houses of my friends, the houses of my brothers' friends. It was ubiquitous, despite me usually wanting to play something else, lol. But other seminal games that were in constant rotation during those days were Melee and Brawl. And there was a lot of overlap. Those games lead the charge on console multiplayer of the time.
Which is all to say, I know a handful of people personally who, should Master Chief make it in, will either pick up Ultimate or even pick up a Switch just to experience it. And that's something I haven't been able to say since Cloud. And just based on the zeitgeist from that time, I don't think that's a phenomenon that will be exclusive to my personal purview. I'm sure some of you can attest to that.