Sure, I can see that line of argumentation, but you’re paying a constant opportunity cost for having cash around
In some sense this is a form of self-insurance rather than paying a “premium” in opportunity cost to the bank each month / year.
Also: I know recommending options to novice investors is considered heresy, but in my experience 1 year puts insuring 50 or even 70% of market value via strike price are often considerably less costly than a 6-8% assumed annual opportunity cost between SPX and some 0/1% saving’s account.
Something that’s automated like this could easily just buy puts with a 1 year window on deposit and paired sell them with equities on withdrawal.
It’s not just coincidence for a checking account, people often need access to cash during market downturns as unemployment generally increases.