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The decision was taken in a hurry because Svensson has not been able to prove himself fully, at least let this 1 season run as he wants. Whether the results are bad or not as long as they are not relegated, then Union Berlin is allowed to take action to dismiss. However only 1/2 of the season running immediately replaced will only disrupt the format that is already running.
Union Berlin is currently experiencing a negative trend, they continue to experience consecutive defeats, even Union Berlin has not won even once in its last eight matches in the Bundesliga. Bo Sevensson failed to bring Union Berlin to the path of victory, he actually made Union Berlin's position even worse. So in my opinion the club's decision to terminate his contract with Bo Sevensson is a wise decision, because in order to reverse the negative trend experienced by Union Berlin, the team must make a significant change, and one of them is by changing the coach.
I was actually disappointed at Bayern Munic for loosing to Mainz but as it is football, such things happens to male the sports interesting and full of surprises. On the other hand, I love the energy with which they bounced back by humiliating RB Leipzig with 5:0. This results shows that Bayern Munic losing to Mainz might actually be a mistake, although, it is a costly mistake. Bayern Munic have good opportunity to win the German Bundesliga this season even though they still have many teams to contend with.
Mainz managed to stop Bayern Munich's record as an undefeated team for 3 consecutive matches. but as you said that when it's on the field then anything can happen. And what I like about Bayern Munich, they don't linger in defeat, where Bayern Munich immediately returns to the path of victory.