The grousing about whether or not a team can be "built around" Ben Simmons is so strange to me. Does it matter for the purposes of the Kings' competitive outlook? If McNair managed to trade for Simmons, he would instantly become the single best player to don a Kings uni since Chris Webber, and he would instantly become the best individual defender in Sacramento Kings history. Yes, better than Ron Arrest in his prime. Simmons really is that good on the defensive end. I'd even argue that he'd instantly become the best playmaker to ever suit up for this team, as well as one of the most physically-gifted athletes in team history.
Is Simmons a plug-and-play superstar that can thrive under any and all conditions? No, but those guys are exceedingly rare in the first place, and the only way the Kings will ever end up with one is if they luck out on draft day. They had their shot in 2018 and missed on Luka Doncic and now here we are, in no position to be picky about raw talent acquisition. If you can buy low on a guy like Ben Simmons, you do it, and sort out the mess later.
And it could be a glorious mess, indeed, for the record. NBA fans are so in thrall to the narratives surrounding Simmons' shooting woes that they've lost sight of the extraordinary talent beyond the flaws. His shooting issues and mental shakiness are not insignificant, but 1) they're also not insurmountable with respect to roster construction around him, and 2) he wouldn't be on the trade block at all without them. Flawed talent is the table at which the Kings get to eat as a small market backwater.
I mean, Kings fans themselves complain every single year about the below-average talent on the squad, and one has to twist oneself into quite the psychological pretzel to convince oneself that there has been any kind of meaningful roster re-tooling this off-season. The Kings are heading into training camp with basically the same exact core players as last season + Davion Mitchell, who happens to play the same position as Fox/Hali. It appears that the front office believes in the notion of configuring them into a three-guard lineup. I'm less convinced, and I certainly would not hope for playoff contention solely on the back of in-house development and lineup experimentation. Even if Fox graduates to official all-star status, Hali takes a big step forward, and Mitchell is as mature and NBA-ready as advertised, the West remains an absolute gauntlet that requires significant talent just to enter the conversation for the eighth seed. If Simmons is available, and if McNair can snag him on the cheap, then he should absolutely do so.
More to the point, Monte has already made it clear that Fox and Haliburton aren't on the table in any kind of trade for Simmons. If the Kings managed to trade for him, it would be an opportunistic kind of deal. It's unlikely to come to fruition, of course, but the reports are that McNair has offered Buddy/Bagley/pick(s) and/or Buddy/Barnes/pick(s), and if either of those packages could somehow get it done between now and the trade deadline next year, then Monte needs to pull the trigger without thinking twice.
There are legitimate questions about fit between Fox and Simmons should the two of them end up playing together, but I don't understand the point of the argument over which of them is "better." Though the OP/thread title suggests trading one for the other, there has been no indication whatsoever that Fox would actually be sent out in any hypothetical trade for Simmons. It's not an either/or situation. The challenge would be in figuring out how to fit them together, which is no small task, but is also entirely possible with some creative thinking. I would add that the team is already attempting to creatively fit awkward pieces together by talking up the possibility of a Fox/Hali/Mitchell lineup. I say try shoving square pegs into round holes with the established all-star/playmaking wizard/defensive wunderkind.