You can be a 40 year old man trying to get a security clearance (and not just for government jobs- many jobs in industry have sensitive government contracts and require clearance) and be rejected because you did not register back in the day.
Whatever conspiracy theory you hold- you can’t anticipate what your future self is going to want to do professionally. “opting out” is a dumb option. As long as it’s on the books, register. The likelihood that the draft is coming back seems smaller than the likelihood you will be hit on the head by a meteor when you go out to lunch today.