It's an opinion. Nobody is stating it as fact. Why would the Premier leagues best striker (I'm assuming you agree) settle for less than a 5 year contract? Also, forever ppl have been saying "English tax" and levy is a genius, stubborn salesman. Now I'm supposed to believe he will let his jewel in the crown go for less than £150m? These are just a guess, an opinion. They seem a realistic guess. As for why ppl think he will decline after 30, they've watched it happen numerous times. Their opinion is they can see a decline like Rooney, Owen, torres, fowler, etc. You can't say yes or no unless you have seen his physio records, but if you think he will, it doesn't make you incorrect until proven otherwise, as with the opposite opinion. Aguero's a one off, a legend!!! Any other player he would of been replaced years ago.
So if asked if I'd prefer mendes, camavinga & isak for the same fee as kane (roughly) I'd jump at it. Its a huge risk over a minor risk. Those 3 players will always pretty much demand the same fee back if struggling due to age. Kane if and I mean if he drops, breaks down is not going to go anywhere on that wage until he's 33.
All opinions and I don't disagree with yours on kane. But I disagree with you jumping down ppls throats calling it "lunacy" when logically your opinion is the highest risk (albeit possibly highest reward).