I don't get how you use the term "output" here. However you put it, the problem was feeding the user's input to ERB.new(). The most general solution would have

I don't get how you use the term "output" here. However you put it, the problem was feeding the user's input to ERB.new(). The most general solution would have been something that accepted any string, but escaped it properly for ERB.new, I think we all agree on that. But that escaping still needs to be done on the input, not the output of ERB.new. If they were able to inject code there, the output doesn't even matter: you've already lost by the time you get a return, the malicious payload has already run, it doesn't matter what its output was.