Let's talk about the opportunity cost of a sideboard card versus Jegantha. As an example, let's choose a high impact sideboard card like Rest in Peace. Because we lose a sideboard slot, we can play a RiP less. (1/x)

Having a free 5/5 in our opening hand will easily win us 1 out of 100 games we would lose otherwise, but let's keep it at +1% for ease. Playing Rest in Peace in the first turns will increase +70%. (2/x)

RiP is not relevant in 80% of the matches. Drawing a singleton (the one we would cut) in the early turns is 20% as an assumption. We only play 60% of our games post sideboard. (3/x)

Under these assumptions, that Rest in Peace is worth a 2.4 game win percentage increase. Just to show how close that is. I don't believe your worst sideboard card is anywhere close to that. I also think a free 5/5 has more impact than a win every 100 games. (4/4)