Richardson isn't someone the Spurs need to "get off"; he's expiring and is a net positive player. A contender might be willing to cough up a pick at the deadline and it's probably worth holding on to him to find out. It's also good to have a veteran amongst all the young players.
McDermott will probably cost assets to move. Other than the veteran part he's the opposite of Richardson: one more year on his contract and a is a net negative player. Maybe he could be a neutral player as a bench shooter on a contender, but that extra year on his contract is probably a deal-breaker for most teams. I don't think the Spurs attaching assets to get rid of his contract is a good move, they have plenty of cap space as it is.