Elite centre forwards are almost a dying breed in football. Haaland of course is breaking all sorts of records this season, and Osmihen is getting a lot of hype in Italy but these are very much exceptions to the rule.
If you look at the top ranked nations - how many of the main competitors - Brazil, Argentina, France, Netherlands, Spain, Portugal, Belgium, Germany, Italy, Croatia have a world class centre forward? I would say none. Most - if they even play a traditional forward - are functional centre forwards who are more team players than anything. Wide forwards have replaced centre forwards as the main source of goals for most teams.
So I don't think the inevitable decline of Kane with no "world class" replacement is necessarily crucial, though it may be a case that England have to think much less traditionally over the hero number 9 role.