Not that this isn't dumb, but what he paid for was the customer base and the domain name, not the software. The software is cheap; acquiring the customers is not.
Rewriting software from scratch isn't cheap, either, especially if you're doing it at the same time as trying to keep the existing code base running. It's practically never a good idea.
But even if it were worth rewriting the software, it's still not what he paid for. He paid $44B because the software is so famous that we're talking about it here, and that would last even through a rewrite. (Assuming the rewrite was done correctly, which it usually isn't.)
Rewriting software from scratch isn't cheap, either, especially if you're doing it at the same time as trying to keep the existing code base running. It's practically never a good idea.
But even if it were worth rewriting the software, it's still not what he paid for. He paid $44B because the software is so famous that we're talking about it here, and that would last even through a rewrite. (Assuming the rewrite was done correctly, which it usually isn't.)