Should NASA Drop SLS In Favor of Falcon Heavy, BFR, or New Glenn?

Now about 7 years into the project and $7B in sunk costs, NASA's Space Launch System (SLS) is still years from completion, with estimated 'per-launch' costs projected at $500M.

At the time SLS was started, the commercial space industry was still playing with model rockets. SpaceX was just coming online with it's then unproven Falcon 9 v1.0, Blue Origin had never performed it's impressive propulsive landing, and the space industry was still stuck in its lumbering, exorbitantly expensive old ways of doing business. If I were on the NASA planning committee in 2011, I would have been thrilled with SLS, and the $500M/launch price tag would have seemed like par for the course.

Given the historically slow pace of space development, I can't fault the good folks at NASA for not predicting just how fast "new space" would make their hardware antiquated before it ever got off the launch pad. But here we are in 2018, with an awe inspiring heavy lift vehicle ready for its maiden flight, capable of reaching payloads close to what we could expect from SLS, and at a fraction of the cost (70%-85% cheaper on a per-launch basis).

Would it not be saner, smarter, and more financially responsible to cancel SLS in favor of simply purchasing the latest generation hardware?

What are your thoughts friends?