It’s not as simple as that. Each block solves the problem of the former block, so to change something five blocks back, you now need to solve six blocks prior to writing the next block, otherwise it’s not cryptographically valid. The resources required to accomplish that are not trivial, and it’s never been done. Very theoretical indeed, in the same sense you could theoretically run through a wall if all of your atoms missed all of the atoms in the wall when you should have collided.