Therefore nobody is ever going to invest the tens of billions required to develop a rocket based SSTO.
If somebody develops an engine that makes air breathing most of the way to orbit feasible, this has a chance of competing a Starship style architecture.
For the reasons you espoused, this is highly unlikely. However "highly unlikely" is more likely than "never".
Atmospheric density reduces exponentially with altitude, which implies that you would need to go exponentially faster to maintain mass flow into your engines and lift over your wings. The truth is that breathing air only gets you a third of the way to space, at best, so you have to have a rocket, and now you're battling that complexity. If your space plane doesn't breathe air, it probably is just better to punch your way out the way conventional rockets do.
Of course, the rocket equation is logarithmic, so reducing the amount of mass you loft gives you an exponential gain. This is true for all propulsion systems to an extent (different constants) but getting into space is the hardest propulsion problem we face. A space plane may or may not be better in this regard (it's been a while since I've looked into that kind of thing, so no opinion either way) but imo the inherent complexity is enough on its own to kill the idea.
The general idea is that you can get much better results in terms of deltav if you can find at least part of the reaction mass from elsewhere without carrying it onboard. Even inert nitrogen is useful as a reaction mass. Another way to get a good result is to use separate sources of reaction mass and energy. Then use that energy to accelerate the reaction mass as much as possible, so that you get a decent deltav by the time you exhaust the reaction mass. This is what ion and plasma thrusters do.