Richardson has trade value.
According to a report from Spurs Talk's LJ Ellis, the Spurs are looking to trade center Jakob Poeltl for two first-round picks. They are also seeking a first-rounder for Richardson. Given his potential value to a contending team, that seems like a fair asking price and one the Spurs might realistically receive ahead of the deadline.
If Richardson is indeed traded for draft capital, that would add to the 13 first-round picks the Spurs have in the next seven years. That would give San Antonio plenty of options going forward, including the ability to trade up in future drafts.
Should the Spurs land the second or third pick in this year's draft, they could theoretically use a first-rounder via a potential Richardson trade to help them move up and select Wembanyama or Henderson. Given Richardson's uncertain future with the team, the flexibility of having another first-round pick undoubtedly outweighs keeping him at this point.
Additionally, the Spurs could receive a young player as a throw-in in return for trading Richardson. With less than 30 games remaining and the Spurs out of playoff contention, a prospect like Romeo Langford could earn some playing time and even prove himself to Gregg Popovich. Therefore, the potential value San Antonio could receive for Richardson is simply too high to justify keeping him around for the rest of the season.