[fiftyfifty]

Minimal orbital speed is somewhere around 25,000-28,000 kph. The fastest air breathing plane every built (SR-71) had a top speed of about 3,500 kph. So for the added weight and complexity of adding an air breathing first stage (wings, air breathing engines, landing gear etc) you are only getting about 14% of the way to the minimal orbital velocity needed, and that would be if your air breathing first stage could match the performance of the fastest plane ever built, keep in mind the SR-71 carried almost no payload. It is far more likely that any realistic air breathing first stage is going to get to less than 10% of orbital velocity, and that would likely be for launching a fairly small rocket. On the other hand the Falcon 9 first stage usually doesn't separate until 6,000-9,000 kph which is closer to 25-35% of orbital velocity, and that's while carrying almost 23,000 kg of payload plus the fully fueled second stage.

[deusum]

Thank you, that context was illuminating.

[rich_sasha]

To make matters worse, even 14% of the speed is still just 2% of the energy which is possibly more relevant here.