Not sure which polls you are looking at, but according to the latest "peilingwijzer" the PVV is losing compared to the previous elections, albeit not by much. BBB might be halved, the current coalition parties together are losing big time.
One reason why PVV may still come out as the largest party is because the rest of the vote is split over so many other parties. There are quite a few sympathetic parties (PvdD, Volt) but in the end they only take votes away from the others.