SLS was also even further behind in development that Starship is... it was supposed to fly before Falcon Heavy. Also the next SLS flight isn't now, but more like 2 years from now... Starship might very well by flying by then(it better be given that NASA plans to use it as the lander).

Not sure where you got the idea F-35/B-21 are questionable benefit over existing systems from(we don't know enough about B-21 to say this, and F-35 seems to have proven itself by now).