Like most "save the webels" ideas, it depends on
Agreed, there is simply no reason for the US not to blockade the CSA. Even if the USN were non-existent when war broke out there would have been a blockade within a year or tow after the US built one. However the USN not existing is a near-ASB event in and of itself.
Like most "save the webels" ideas, it depends on individuals who were demonstrably a) pretty capable individuals and b) successful, historically, to suddenly become drooling idiots.
And, as repeatedly demonstrated during the actual conflict, (and unsurprisingly, given the 3-1 or even 4-1 population differential the loyalists enjoyed over the rebels), even if someone important stopped one, they had a deputy who (generally) could and did step up and kept up the fire.
As a thought experiment, consider that even if someone important - in the Lincoln Administration, the Army, the Navy, or even in a key state - "departs" because they stopped one, or fell off a horse, or died of cholera, or whatever - they all had someone (several someones, in fact) who would have replaced them.
As much as Lincoln and Grant were amazingly capable individuals, truly the right men at the right time, is someone really going to argue that Hamlin or Seward or Sherman or Meade were not pretty damn capable, as well?
One can, but it veers into the realm of the fantastic, and if one wipes out everyone of note in the US at this point, it gets rather fantastical.
An obvious point of comparison are how many effective general officers the US had in the field by the end of the war who, demonstrably, were capable of effectively leading an army or army group in the field against the best (or worst) the rebels could provide to face them. One can include Grant, Sherman, Meade, Thomas, Sheridan, Ord, and Canby on the US side, presumably, before one even gets to the corps commanders; on the rebel side, who's left?
There's Lee. Okay, that's one. If one is
very charitable, maybe JE Johnston; maybe Kirby-Smith and Beauregard if one squints
really hard.
2 to 1, at least; probably closer to 3-1, since the US corps were pretty close to a rebel army in strength by 1865.
Quantity has a quality all of its own; quantity
and quality is pretty close to undeniable.
Best,