Dan Ticktum's just fallen off the programme, so that's a factor. There is a Superlicence-ready Red Bull driver though (Patricio O'Ward) in Indycars, however, so if I was Pierre I would not be quite as sanguine as he is probably needing to look in order to maintain the balancing act that is his career. Not yet, admittedly, but once the Indycar season is over, Patricio would be available to try out in a Toro Rosso if Gasly isn't pleasing Marko and Tost.
The fact that Red Bull have had to "borrow" their drivers for a while (Verstappen was picked up by the programme 6 days before he was signed to their F1 seat, so Red Bull can't claim credit for developing him) says that the programme stopped working as an in-house development system years ago. It's going to take an internal restructure to change that.