I think what the poster meant, at least as I interpreted it, was that once you've done your time and paid your "debt to society" you are once again a free citizen with all the rights, privileges and responsibilities attending such status. Certainly the framers didn't intend to have some star chamber decide the fitness of each person and indefinately confine those deemed unfit. I believe you've misunderstood the comments.
There was no assumption back then that "once you've paid your debt" by serving a prison term, you are automatically considered as having the same legal status as if you'd never committed the crime. They had common sense back then. They knew that people who have committed serious crimes are very likely to do so again, and they did not imagine that either the law or other individuals would deal with these people as if they had never committed a serious crimin. They also knew an insane or retarded person when they saw one and had no thought of allowing such persons to run around with guns.
The issue of how the definition of "felony" has morphed is more valid. Back then, things like having a gun when your "permit" for same had recently expired, certainly didn't constitute a felony.