4% * $50 trillion = $2 trillion in just interest per year. The US Government will be at roughly that debt level perhaps within a mere 12 years at this point (assuming one normal recession in that time).
So as you can see, the US Government can afford no meaningful climb in the average interest rate it's paying on its swelling debt. Normally as a massive borrower gets more shaky in its finances, the rate it would expect to pay would climb at an accelerating pace with its fiscal implosion. To avoid that fate, the Fed has to step in and become the primary buyer of the garbage US Government debt (which will be endlessly recycled as they will never pay any of it down now). The financing problem with the US Government is so enormous at this point that there are no other nations or private pools of capital large enough to keep up with it on a consistent annual basis, currency debasement - eating the value of anything held in dollars, including most US assets - is all that is left to fully fund the government (and the entitlement costs will only keep spiraling higher this decade and next).
The stock market is a laughably trivial concern next to that problem. Which is also why one of Biden's immediate priorities is raising taxes (if they don't, the Democrats know it'll put the welfare state & entitlement programs they've put so much effort into building over many decades into jeopardy sooner (collapsing demographics may take care of that regardless)).