This sure is a sad and scary development. But it's worth considering the reasons behind it. If people feel that their only alternative is "the Alternative" it also means the failure of everyone else: the mainstream and the actual alternatives (provided they do exist).

Which political parties represent the interests of Big Business, corporations and investors, rather than the average individual employee?
The FDP? - Most certainly.
The CDU? - Also apparently.
The SPD? - Them too, allegedly.
The Greens? - Not sure, quite honestly. (Though I wouldn't put it past them.)
The BSW / Linke / PDS / SED or any of its incarnations? - Doesn't matter, really.

If anything, I see the extreme left very similarly to the AfD, meaning that they're merely a fringe option, to indicate voters' disillusion with the rest. Kinda like the APPD used to be: not meant to win and govern, but instead ring the alarm bells so the mainstream would get its act together. And it's still not too late for that.

But now that the AfD has actually achieved these results, it's worth pointing out: They also represent money (and those who have lots of it but want more still)! As you mentioned, their tax plans reveal their intentions. But doesn't Alice Weidel also have close ties to Goldman Sachs? Yeah, so much for the "alternative" then. So maybe those teens got it right, and the best option is the Linke, or even better, whichever alternative ideas are lumped together under "others".