On the plus side, Presbyterianism has thoroughly taken root in the US. Though it is much less strong today than it was in colonial times, particularly in New England, it has been making something of a resurgence thanks to the work of some very visible contemporary proponents of Reform (and particularly Presbyterian) theology.
More realistically, the churches in the USA that formally adhere to the Westminster Confession and maintain a Presbyterian form of church order in the authentic Calvin/Knox tradition number less than one million people in a nation of 300 million plus.
There are also the 3.5 million members of the PCUSA, which I don't believe Knox would recognize as Christian, let alone Presbyterian.
it has been making something of a resurgence thanks to the work of some very visible contemporary proponents of Reform (and particularly Presbyterian) theology
While there has certainly been a resurgence of quality theological work among Reformed scholars, and a resurgence of partially Reformed views among many Southern Baptist and independent congregations, there really hasn't been growth at all in the number of churches that follow Presbyterian church order.
A good example would be R.C. Sproul, who has sold very well as a popular theologian - but is himself running an independent congregation that is not part of any presbytery.
J.I. Packer is an Anglican. James White is a Reformed Baptist (a denomination with fewer than 50 congregations and not part of a presbytery as the name implies).
The one Presbyterian denomination that seems to be producing reputable theologians is the OPC, which is tiny.
I’m well aware of the resurgence of Reform, however, I was surprised at lunch with my Reform friend when he sounded as if he was embracing something that may be called “Catholic Lite”.