What if your feet are already ruined by bad shoes and bad technique?

There's no such thing as "ruined". There's just a matter of how long and how gradual are you willing to work walking/running barefoot into your life. Why take such a fatalistic approach to your feet when it's so easy to just take your shoes off and see for yourself?

I imagine this question comes up most from people who are scared to death of being barefoot for fear that their custom orthotics are the only thing that support them. This may be true at the moment, and this is why you should be careful.

Careful to ease into things, careful to listen to your body and be aware of how it is reacting.

You aren't ruined.