As the comments show, this public letter only gives fuel to Trump's "anti-establishment" voters. I don't see how it accomplishes anything else. If they're truly working against Trump, this admission only reduces their ability to do so, by making him suspicious.
Imagine if Trump had issued a press release through Sarah Sanders that there are insidious forces within the administration, working against him, responsible for the administrations apparent self-contradictions and failures to enact his policies.
We'd call it old news, because he's always gone on about "the swamp", "establishment", etc. But here is a supposed part of the establishment, verifying it all. It's 1984's 2-minute hate.
The anonymous author criticises Trump for unpredictability and "little affinity for ideals long espoused by conservatives: free minds, free markets and free people". Unpredictability is the "virtue" his base voted for, and the rest is a meaningless "attack" by the establishment on Trump.
When it comes to actual campaign promises, even this nameless villain acknowledges Trump's "victories":
Don’t get me wrong. There are bright spots that the near-ceaseless negative coverage of the administration fails to capture: effective deregulation, historic tax reform, a more robust military and more.
These are terrifying times.
If this is real, then the dissenters need to come forward with the evidence necessary for impeachment - as they claim to have. This essay only strengthens Trump.