"A serious advance in nuclear, or better yet fusion, propulsion might start a trend where space travel could start moving as many people into space as we do people on airliners (don't know about "billions.")"

I don't think there is a need of serious advance in nuclear, without any advance I believe 900 Isp is possible.
A 900 Isp, at least halves the fuel needed, I believe. Yes, we don't have such a rocket, but I believe there has been proto-typing which has proven the concept.

But there other routes other then fission or fusion [though those are promising].
For example using lasers that heat air which pushes a rocket- now laser are expensive and you need much bigger lasers then available, but it could doable. Or as just a guess, instead burning air maybe you burn something that has higher gas temperature and is a light material, carbon or silicon compound?

Leaving aside the exotic technology of fission, fusion, beamed energy, sram-jets, etc.

One has cruder brutish stuff- you could launch from higher elevation. With balloon type platforms, 100,000 ft would be possible, but maybe this could pushed to 200,000'. Now, of course these balloon platforms would need to be big and would be expensive. But if you launching 10 to 100 launches a day, they could be relatively cheap. And since they would need to be big, you could make them even bigger, and launch rockets horizontally from Mag Lev track. Or the balloon could hold up/stablize "a pole", have rocket on a vertical Mag lev track that runs up pole and accelerates rocket vertically.
These things could still use chemical rockets for power at starting at mach speed [mach 4?] and from +100,000'. Or essentially you removing the first stage rocket and using rocket nozzle designed for vaccum [as second stages generally are]. So reducing fuel use by at least 75%, and somewhere around a 60% reduction in total energy cost.

There is another way. One essentially does a suborbitial trajectory and you dock/catch the ship and then accelerate it. Tricky, but it could be done. Now, what accelerate the rocket could be various things. Mechanical tethers is one thing that might be possible. Now if these mechanical tether are used for stuff which is being imported to Earth, it will have "surplus energy" to bring things up.
Now if anything starting with a "N" word is banned, from Earth's troposphere or lower, a nuclear tug could provide the acceleration.
Or if energy and rocket fuel is cheap in space, one could simply use chemical rocket tug to grab it- that should still reduce total fuel used, or at least, use the cheaper space fuel than earth fuel [if it it costs 100 per lb to lift anything from earth, then rocket fuel made in space which cost less than $100 per lb is cheaper than "Earth fuel"].

Now any of these things doesn't have to be the "one solution", I'm sure other things are possible and "all of the above" could what happens.