In 2020 Ingles, Mills and Delly will all still be on NBA contracts in the US$10m/year vicinity, so NBA teams, at least, have have gambled that these guys will be able to contribute in a few years time.
If any of these guys come off the bench, someone must have developed (or proven themselves) to be significantly better. Experience and chemistry mean a lot, so as good as Simmons is, I'd be surprised to see Simmons take over starting PG duties in 2019.
Although the game seems to be moving away from relying on a single point guard to initiate offense. Imagine having 3 skilled passing playmakers (Delly, Ingles, Simmons) alongside a part-time PG (Mills), a defined "starting PG" becomes redundant.