As the 2019 Williams FW42 is showing, even teams with illustrious histories and good people can make a slow car.
McLaren did the same in 2013 with the MP4-28. The MP4-27 had won the last two races of 2012 in USA and Brazil, but the team plummeted to the midfield the following season. And let's not forget the car that never raced - the MP4-18 of 2003. It took McLaren two years to climb back into a winning position with the MP4-20.
It's easy to fall off the cliff of performance, all it takes is a simple mistake. The challenge is to identify that mistake and then have everyone agree that is in fact the cause of the loss of performance. This can be the hardest part. Once you've done that you can formulate a program to dig yourself out of the hole. Ironically, the loss of performance is often created by trying too hard to make a huge jump in one go.