The downside is that you lose the original and can't refer to it anymore. If (like OP) you are writing a compiler, you may need to spit out error messages, and will want to be able to say "line X, column Y", or even just print out the original line itself.
And no, you can't re-escape your unescaped text, as you can't do so losslessly in all cases. For some, the unescaped text might not obviously need re-escaping (e.g. the original text had a unicode escape for an emoji; you'll "forget" that after you unescape it).
And no, you can't re-escape your unescaped text, as you can't do so losslessly in all cases. For some, the unescaped text might not obviously need re-escaping (e.g. the original text had a unicode escape for an emoji; you'll "forget" that after you unescape it).