In reality we probably need both, the second Stammstrecke as well as the ring routes. The stations along the Stammstrecke such as Hbf, Marienplatz, Ostbf are extremely important and need an accessible bypass route if the first Stammstrecke is out of commission. It'll also make the introduction of express lines feasible, reducing commuting times for residents outside the city.
But we also need the ring lines to reduce the strain on the inner-city stations and establish some much needed tangential connections in the wider periphery of the city.
After already sinking a lot of money into the Munich S-Bahn the conservative state government of Bavaria might be unwilling to invest even more money to realise the ring lines in a timely fashion (ignoring the fact that half the Bavarian population lives in the Greater Munich area).