Vulcan, New Glenn, and the Falcon Heavy really aren't in the same league as the SLS, at least as far as lifting capacity is concerned.

I think the SLS is an excellent use of NASA's time and money at the moment. They're using proven technology to build a system that they can either:

- fall back on in the event that commercial companies aren't able to deliver something comparable

- still act as a viable launch platform if cheaper alternatives are somehow disabled (e.g. another Falcon failure)