Kane is not doing full 90 minutes tho. His work rate has been terrible compared to other forwards of teams who are winning things.
I do see your point but the situations are very different. Kane has already proven himself over and over again. We know that if he can only give 90% for most of a match at the moment or if he tails off after 60 minutes, he's worth persevering with anyway because he needs that match fitness after a very serious injury break.
And even while he's at 80 or 90%, he can still score match-winning goals and make the difference. Despite his entirely understandable suboptimal fitness at the moment, he's bagged 4 goals since the restart (very unlucky not to have at least 5) while winning 5 points for the team.
Nobody could do his job better at the moment. Nobody else on the team has scored more than 2 goals since the restart, suggesting that everyone (including most of the rest of the league) is re-finding their shooting boots (only it's much harder for Kane).
With the current fixture density, staying at 80% may well be what he's instructed to do. In fact, I'm convinced that Kane is indeed receiving game-by-game instructions on energy conservation. He needs to manage his stamina coming off the back of his worst ever injury to play a game every 2.5 days! It shouldn't really be happening and yet it's also the most sensible thing to do for the club under the circumstances (if we want a hope in hell of Europe).
To some extent, I think we should just be happy that Kane's come back from a potentially career-ending injury: he's still able to sprint, able to at least run for most of a match despite inhuman fixture congestion, and now he's scoring like he used to. The club's not a charity but if anyone has a case for being allowed the rest of the season as a write-off, it's Kane (fortunately, he isn't treating it as a write-off and his stats reflect that). Ndombele is a long way off deserving the same latitude -- in no small part because we don't know him well enough to know if he'd still try his best like Kane.