I wouldn't call it throw away ticketing when we bought a more expensive replacement ticket, also from UA.
You might not, but UA would. You
didn't buy a more expensive replacement ticket. The cheapest one-way fare from France to San Francisco is almost certainly more expensive than UA's internal accounting value for 70K MileagePlus miles.
Furthermore, UA knows that many Basic Economy passengers decide to change return dates and buy new tickets. That has never been considered throwaway ticketing as far as I know.
There's a difference in intent.
Anyway, long story short, it doesn't matter. The answer to your original question is that UA is not going to waive any fees or do you any other favor due to cancelling the return flight in advance. In fact, the only reason to cancel early would be if the two return flights are on the same day or otherwise impossible to fly -- if UA detects impossible bookings, it reserves the right to cancel one of them. Otherwise, wait and see if you get a lucky; perhaps a schedule change or a delay will allow for a refund.