> and both the Atlas and Delta both have very specific high-energy orbits that the Falcon cannot offer—even when it is used fully expendable

That's why Falcon Heavy exists... besides, Atlas and Delta are retiring, they cannot get new launches anyway.

> Other than that, they have their proven track record of decades of reliability.

The currently active version of Falcon 9, Block 5, has a 149/149 track record. Falcon Heavy is at 5/5. All these launches were made in the last 6 years, which is a much better indication of current performance than launches from 1990.