I would welcome you to publish the physics which shows that kinetic energy is not the sole dominant factor in getting a payload into orbit.
Of course this has always been the prime rationale for multi-stage launch, as was first realised in the mid-twentieth century and ever since then multi-stage has been the only possible way to attain orbit. That Zootycon is using the same rationale to try and justify SSO technology over multi-stage is - what, ironic perhaps? SABRE makes it technically possible but it doesn't change the proven economic advantage of multi-stage.
Second-best is a subsonic COTS (commercial-off-the-shelf) mothership with SABRE upper stage. As British Aerospace found donkeys years ago (and contrary to Zootycon's dreams), this is the cheapest and lowest-risk orbiter to develop. And as Pegasus has amply demonstrated, a conventional mothership brings operational flexibilities which have high value in themselves.
Better is a suborbital SABRE-powered mothership. It might even borrow a trick from the Shuttle, with the orbiter's conventional rocket engine providing boost for takeoff. But that's two specialist craft, with twice the development cost and the risk squared. Let's do it the easy way first.
You don't even need SABRE. An atmospheric mothership, suborbital rocket second stage and rocket orbiter would have been viable fifty years ago - think B52 plus X-15 with disposable orbiter hung beneath, then refine. But that refining triples the development cost, so nobody ever dared go there.
Exercise for the student: Taking the Virgin Galactic two-stage suborbital launch system as a starting point, how is it best developed to give orbital capability? SABRE for the mothership? SABRE for the second stage? A new rocket third stage? Richard Branson would love to know!
But if you are willing to set aside orbital flight for now then, whether or not you believe that SSO will ultimately triumph, a sub-orbital Skylon is an obvious next step.