Again, I can point at my own country and point out that CDA (similar to the CDU) and the PVDA (similar to the SPD) are shadows of their former selves. Both parties pretty much got wiped out in recent elections. PVDA combined with the green party for the last round of elections and managed to recover a little bit. But the CDA which once had the same status as the CDU in Germany of being the defacto governing party was all but wiped out this election round. 5 seats in a 150 seat parliament. They've become a fringe party. NSC founded by a former member managed to grab 20 seats.
That's a party that did not exist until a few months before the elections. And they were polling in the lead at 35 seats at some point. A completely new party. Their main competitor is another new party that peaked a few months before the elections but didn't manage in the end. The polls kept changing right up until the elections and the outcome caught a lot of people by surprise as, evidently, some 10-15 seats ended up shifting in the last days/hours. VVD (which supplied the prime minister for the last ten years), was expected to win, but ended up losing badly to a populist that had previously been written off and made a miraculous recovery and won over two other parties that both been polling in the lead as well.
This is just to provide a little insight into how things can change and how there are no certainties in politics. The VVD never lead a government until the CDA imploded after their last government in 2010. Now the CDA is wiped out and the VVD is at risk of being wiped out as well. Voters have flocked to new parties that didn't even exist until a few years ago.
The same dynamics are at play in Germany and elsewhere. Lots of Germans are not really committed to voting on any of the traditional parties. And the ones that still are, aren't feeling that great about it. Everybody assumes that everything will just revert to people voting for the same parties because nothing ever changes in Germany. But when change comes, these things stop being true. If CDU/CSU ends up forming a government with the AFD, they could end up paying a heavy price. That's exactly what happened in the Netherlands in the last 20 years. It completely wiped out the CDA. This was unthinkable until about 20 years ago.