Using a commercial launch vehicle reduced the cost quite a bit (saved a billion on the vehicle itself and another billion by not having to design the payload to pass SLS's much more rigorous vibe checks). The $2b saved is about 10% of NASA's annual budget. The next mission to Titan has a budget of around $3b, for reference.
On top of that, there's no guarantee SLS would've actually been able to launch on schedule. The program has had a lot of setbacks, to put it lightly, and has only launched once so far (6 years later than it's original ETA). There's additional rockets in production but if it came down to it the manned SLS missions would probably get priority for political reasons.
It saved far more then 1 billion $ for the launch vehicle. That number is just more SLS propaganda. If you actually do the research its far more. At least 1 $ billion more and that is the very best case, not accounting for any amortized development. In reality its likely considerably more.
On top of that, there's no guarantee SLS would've actually been able to launch on schedule. The program has had a lot of setbacks, to put it lightly, and has only launched once so far (6 years later than it's original ETA). There's additional rockets in production but if it came down to it the manned SLS missions would probably get priority for political reasons.