I reject your premise. These are independent events and should be viewed as such. The merits of selling Kane can and do stand on their own.
Just wondering, how many more games like the previous 4 will it take for you to say "Damn, perhaps we should have sold Kane"? I guess a better way of asking is can you envision a scenario where you will view the keeping of the player to have been a bad idea? Say from this day on he never scores another goal for us, his availability is below the Anderton-line, he accumulates red card offenses in 50% of the games for which he is available, has alcohol and granny scandals monthly, and is tainted by whispers of match-fixing. Would you then concede that it is regrettable that we didn't sell him? Serious question to establish a baseline. If you say no, then you've abandoned all reason and there is no argument to be had on the matter.
I think in the hypothetical world of a message board you are right but in the real world of Daniel Levy and Fabio Paratici we need to consider what will be done with the money if we did sell Kane, especially for anything. If selling Kane means we end up with a Bergwijn-Son-Moura front 3 then there is no scenario in which it makes sense to sell him, at least no realistic scenario.
As to the second part, really right now I would almost say it is a mistake already if the decision is to keep Kane and not build around him. If the plan is to build for the future with potential buys then sell Kane and use that money to buy quality potential buys and numerous ones so that when we see that potential full filled it isn't a team of 2-3 guys like we have now but a team of 8-9 guys that can actually compete.
The problem I currently see, which the Kane scenario is only part of, is that there is zero plan. Zero big picture thinking and less than zero thinking in terms of how to build a team. We are trying to win now, but not making moves to win now that doesn't work.
For me the best way for us to have any success, and it is likely to be little success, is to keep Kane and pair him and Son and hope their individual excellence (and yes Kane still has individual excellence) can overcome the lack of talent in the squad, lack of depth and mediocrity in the manager.
If you asked me my ideal (and somewhat realistic plan realizing Levy is not going to buy the players needed to compete with the top 4 teams) I would bring in a proper DOF or manager I am not too hung up on the power structure so long as someone or some people that are competent are making the decisions not Nuno, Paratici or Levy, sell Kane and many others (likely Son as well) and build the team from almost scratch.
I have always said I am fine selling Kane but I need to see a plan of what to do. Saying (not you necessarily) that we are better off with Bergwijn-Son-Moura is not a plan, that is a relegation fight. I also think saying buy young players is a techinically a plan but a poor one. Replacing Kane with a lesser version who is younger (Toney, Vlahovic, even Martinez) to me isn't much of a plan either, it just makes us a younger team with the same exact problems.
The issue we have now with Kane and Son is that they paper over the cracks which prevents us from moving on to make changes we need because Levy is scared what happens when his security blankets are gone and at the same time if we fuck up moving on we lost our security blankets and it could get ugly.
Not sure if any of that answers your questions but the short answer is yes there are certainly times to sell Kane but that comes with a lot of qualifiers for me.