The San Antonio Spurs shouldn't be let the young core hold them back
Going relatively hand-in-hand with the previous point, no matter who the Spurs decide to pick in the draft, there will likely be some overlap with one or more of the young players on the roster in terms of their role on the team. That, however, shouldn’t deter San Antonio from picking them, particularly if they feel that they are the best player remaining on the board. In short, talent comes first and team fit comes later.
In such a talented draft class, a player selected 12th in this year's draft could very well go on to surpass another young player with an already defined role on the roster. For example, if San Antonio were to select Kai Jones, he could conceivably surpass Drew Eubanks and eventually Jakob Poeltl for a spot in the starting lineup if his development goes well.
Although fans have grown attached to the Spurs' current young core of Johnson, Walker, Vassell, Poeltl, Samanic, and Murray, it's highly unlikely that they're all a part of the team's long-term future, especially if the Spurs have an opportunity to trade for a higher pick. Let's not forget how George Hill ultimately left the Spurs (and who they gained in return).
Assuming that the Spurs continue to draft in the lottery for the next few years, I would go so far as to say that it's more likely than not that some of the young core will eventually be replaced with more talented players. If that opportunity arises this year, the Spurs shouldn't think twice about using their lottery pick to potentially upgrade a position.