It's not that comparable because the Lib Dems have never had a 'safe voter bloc' they can rely on, which I assume the PvdA formerly did given their history similar to Labour or the SPD. A collapse of a (once) class based party like that is more noteworthy in my opinion.
Before the war, when they were the SDAP, they were one of the four pillars of Dutch society (the others being the Catholic Church, the Reformed Churches and the liberal/"general" pillar of business and intellectual interests). Party officials used to boast that their voters put "SDAP" down as their religion on census forms. The only other times since the First World War that they'd polled less than 20% before this were in 1918, 2002 and 2010.