Harry Kane is loyal to his team Tottenham, but still, we know any player will leave his team one day and for a 30-year-old player like Harry Kane this season is a good chance to leave Tottenham and join a better team to achieve whatever he wants, otherwise, he can't he this chance in the next season. Also, Tottenham can earn a good amount of money for selling this player from this deal.
Few players under 30 get the opportunity to join a prestigious club like he just did. He should have left Tottenham years ago, but he didn't likely because he feels he is indebted to them. I believe now is his last chance to make a move out, or else he may not have the opportunity to leave again. He is very good and has the scoring ability to play top football for three more seasons. All teams desire to sign young players in order to retain them for a long time and to generate revenue in the event that they decide to sell them in the future.
Dortmund hope Sabitzer to fill Bellingham's place nicely. There is a big difference between these two players. While one of them is more of a very young but really skilled midfielder the other one is a much more experienced decent level midfielder. Maybe Sabitzer's talent level isn't high as Bellingham but I believe he is quality enough to keep Dortmund midfield strong. He had a short journey at Manchester United and now he is back at home which is the Bundesliga.
Bellingham can try to fill the void left by Sabitzer, who is also a good player, but he won't succeed entirely. Bellingham will undoubtedly be missed by the squad, and it will take some time for them to get used to. Sabitzer can be managed for the season or, more likely, until they sign a fresh player of his calibre. Finding a good successor wouldn't be difficult because Dortmund is quite adept at spotting talent.
David Raya and David de Gea are the only alternatives to be rumoured for Bayern Munich to replace Sommer. Actually it depends on whether he leaves or not. For now as you have also mentioned his name is called with Inter. If Sommer goes there there will be a serious need of a new goalkeeper.
Since Andre Onana left for Manchester United, Sommer has been linked to Inter Milan. Inter Milan has been looking for replacements, and Sommer was their first choice. Although Sommer is a decent goalkeeper, I doubt that Bayern are happy with his performance thus far, which is why they are not concerned about losing him. If Sommer leaves, Bayern is a top team, so they shouldn't settle for a goalkeeper like De gea. If they truly want to return to their former competitive selves, they should look elsewhere for a young goalkeeper with talent.