1 - Chance but certainly not a guarantee. We haven't bought a have a manager from Australia -> Japan -> Scotland who has never even coached in the PL, and we finished 8th last season with Kane. To qualify for the CL we'll need to be better than at least 2 of United, Woolwich, Chelsea, Liverpool, and Newcastle. I'd say even with Kane its more likely than not that we don't qualify for the CL.
2 - Players would be "bedding in" to a system/style that's different from the way in which we'd be playing next season, so then it's another trial next year on whether or not the players can adapt/are suited to the new squad.
I understand people want Kane to stay. I WANT him to spend his entire career at Tottenham. But he's pretty plainly signaled he doesn't want to. He gave an interview about wanting to play at City, then went on strike to try and force a City move, then pouted for 6 months that he didn't get to move to City. When it became clear that City were going for Haaland and that chance was gone, he then lumped it and moved on - for a time. It seemed before this summer that he was happy to run down his contract and move on a free.
Bayern came in and have apparently offered him something he's happy to take to move there instead. If we don't make it work, he's leaving on a free next summer. He's been offered new contracts with increased wages and turned them down. Harry Kane will not play for Tottenham in 2024-25, period. All arguments for how we should deal with this situation that try to leave that door open, are simply grounded in a wishful fallacy.