First up, no Laker GM will be willing to trade Luka Doncic. He is going to be a Laker around which they will build their basketball squad over the next decade and a half, if need be.
Second, it is inherently stupid to give up on Castle and/or Harper so soon. The Spurs are incredibly lucky to have such talent who are playing far more mature than their ages and are far ahead of the curve for sophomores/rookies in many aspects of basketball and especially the more difficult ones such as playmaking, defense, positioning, finishing at the rim, assisting etc. Shooting is their only weakness and that is something that can be worked upon over time. It is a tribute to their other abilities that they are net positives on the floor despite poor outside shooting.
Thankfully unlike immature Cassandras in the fanbase, franchises employ mature talent evaluators who understand basketball well. Every other team in the league would be jealous of the Spurs' situation with Wemby, Castle and Harper. No analyst worth his/her salt would suggest an outright trade of one of them even for a MVP candidate like Giannis because of the downsides of such moves (different timelines, fit, health of the superstar etc) as opposed to the positives of having outstanding young players on a rookie-scale contract growing alongside a generational player like Wemby.
It is a testament to Castle that he is already spoken of as a possible All Star (and he is also listed among the Top 15 choices in the WC by fan vote) despite his 3P Shooting woes. And while Harper has recently hit a rookie wall, his performance as a slasher and finisher has brought comparisons with elite NBA players like Kyrie Irving/ Cade Cunningham etc. Plus Harper and Castle aren't just theoretically good players. They have been part of a devastating guard trio along with All Star De'aron Fox that has helped the Spurs maintain a hegemony over the reigning NBA champion OKC Thunder who threatened the all time NBA regular season win record this year.
Clearly those of you who are asking to trade one of them are idiots.