From my understanding opposition is something very important in democratic system, it servers as a controller of the ruling party and put on it some pressure to do actually something useful.
In addition opposition is something natural, people tend not to agree on everything and should have right to express that, also through democratic system mechanisms.
Since there was no opposition in Germany, it has been created, this was a natural process. The fact that AfD has raised as main opposition force simply tells that ruling parties simply missed something people are afraid of or need. Pointing out failures of the ruling party is the role of the opposition and that happens in Germany now.
Someone might not like AfD (I don't), but it appears it attracted some people, if someone plans to ignore them, this is not a good idea.
Not that long ago someone decided that it is not a problem to ignore humiliated people who came back home from battle fields of I World War straight into poverty, however there was a certain Austrian painter, who managed to attract them very skillfully. The rest of the history is well known.
That is exactly why there is so much outrage. People have been predicting that history ryhmes for a long time, but now it is actually starting to happen. The CDU's decision to rather cooperate with the AFD than supporting a socialist coalition is a paradigm shift. Yesterday's decision has shown that the nazis are back as a bona fide political power.
"Den größten Erfolg erzielten wir in Thüringen. Dort sind wir heute wirklich die ausschlaggebende Partei. [...] Die Parteien in Thüringen, die bisher die Regierung bildeten, vermögen ohne unsere Mitwirkung keine Majorität aufzubringen." A. Hitler, 02.02.1930
The quote is from 1930, which is still at the beginning of Hitler's rise to power. In it, he reckons that the NSDAP had now become a major political force since the established parties in the state of Thueringen were, for the first time, unable to stand up a government without the votes of the NSDAP. This is pretty much exactly what has happened yesterday.
What's worse is that the "christian" conservative party, which is the largest centrist force in Germany, has basically given their tacit approval to all of this. They were the ones who teamed up with the nazis to put a center-right candidate into power. Without the votes of the nazis, he would have lost the election. Mind you, they did this after years and years of political campaigns claiming that they'd never cooperate with the AFD. That promise aged like milk.
To put it into perspective, it's a bit like the GOP of South Carolina teaming up with the local KKK to put a new senator into office because the KKK controls over 20% of the votes in the state. Imagine that for a second. It's scary...