Even as Kane's biggest fan, I get what you're saying. Potentially, Kane could be wasted on a rebuild, in the sense that if you've hypothetically got a mediocre squad for the next few seasons, keeping Kane alone (or any one player) probably won't get you top 4. And the money gained from selling him could be distributed across the squad to raise the average level instead of having a completely top-heavy XI.
I wouldn't want to see Kane spend the rest of his theoretical peak in a team where he definitely won't win anything (all of this is hypothetical -- I don't know what's in store for Spurs!) and I think it would be magnanimous of supporters to wish him deserved success elsewhere if success is indeed off the table here.
Although I think it would be a move that a lot of fans would come to... not necessarily regret, but certainly a loss that they would feel (a la Trippier times ten) and I don't see the club getting a readymade replacement anywhere near as good, I don't think you should be chewed out for your opinion. I think it's something that really depends on your view of the club's medium-term trajectory: if Spurs are going to be back in the ascendancy next season, I definitely wouldn't sell him, but if the current level is a more accurate representation of the future, the net gain from selling him might be greater for all parties.