I wonder in what sense these agreements are enforceable when they cannot prove that people actually agreed with them? Hard copies of contracts are kept for a reason, I always assumed.
When there's a EULA or similar, the company is trying to form a contract with you, usually with lots of unfair terms.
If replacing the text with your own and clicking a button that you renamed from Agree to Disagree voids the whole thing, well, mission accomplished.
There would only be a problem if the law says that not only does your modification not form a contract, but that you acted in such bad faith that you actually still somehow agreed to the original contract.