In his statement as to Kolisi's move to the Stormers Dobson said his role is more of a mentor than he is as a loosie. Bearing in mind that the Stormers have by some distance the best loosie combination of all SA franchie team there is obviously no paticular interest in Kolisi as a player - but more particular in developing him as a coach.
In this type of situation Erasmus influnce on decisions taken is massive since all he Franchise and provincial team reports to him and are influenced by Erasmus's points of view,
So if Kolisi is unlikely to ever captain the Stormers team and that means he is soon to be replaced as Springbok capttain, Kolisi has been he Springbok captain since 2018 - ie more than 7 years. Erasmus will look a repalcement of the captain with a player with long playing career ahead of him and in that regard Sacha will meet all requirments, Fact is Sacha has been a protegee of Erasmus for years and that means he is clearly being used based on Erasmus viewpoints on issues.