Imagine an scenario where all the minor parties drop out and only Linke, AfD and CDU make it into parliament. What happens next? (also assuming Linke does not get a majority of its own)
If it looks like a party is going to fall below the threshold, it's not uncommon for it to get lent quite a lot of votes by tactical voters. See for example the 2017 federal election, where the FDP ended up way above where most polling from a few weeks before had put it and not down to any efforts on their part.
I'd argue the most dangerous spot for a party to be polling in is 6-7%, whereas if it's getting 4-5% there's a very high chance it will get in.