What commissions are due a commission only sales rep (independent
contractor) upon termination of their contract, with either party
terminating?

Just a layman's opinion but, unless you have specific provisions in
the contract, there is no commission owed to the former salesperson.

For example, would you have to pay commission to a former
employee/contractor for the next 50 years on sales contacts he/she
brought in in a single year?

I hope this helps.

Have a good day.