Who accumulates decades of legacy code?!

Real companies do. The moment you deploy one line of code, it's legacy. It goes from there. Soon you have to build systems that interface with other systems you'd rather were better architected and designed, except you have to deal with them as they are. Then your product becomes one of these, and with no need to maintain or expand it for a long time, it rots a bit, and now someone has to pick it up or interface with it, and your product made things more complex, and the complexity can't be magic wand waved away.