If a constitution had a pro slavery clause and a party is trying to change that we would consider them heroes, so why is it such a bad thing to change the system if it is failing to provide for the people?
The constitution protects people from voting their rights away forever just because they look for a quick solution. This has happened in the past, and to prevent it from happening again, the constitution has been strengthened to the point of banning anticonstitutional parties.
What the AfD struggles with is that the constitution enforces democracy. And while this is obviously an instance of the tolerance paradox, prohibiting people from removing democracy is the only way to ensure their rights and freedoms, and most importantly their power to control the government (by voting).
Constitutions exist to provide friction to fast-moving populist votes. They prevent the two wolves from eating mutton after the vote. They are not immutable; they are hard to change, and can limit simple-majority votes.
I read into your question, that they should be allowed to do that. The question is, why should they?