Besides, I'm not talking about the foreseeable future.
I'm talking about the future. Ultimately, say a century
or two down the road, where will our energy come
from? Solar power is the obvious conclusion.

So the oil has run out - where do we get our energy?

Wind power, tidal power, geothermal power, Earth-based solar power, and
fission reactors (more of which are being built right now) are all far
more cost-effective than anything that has to be launched into space,
maintained in space, replaced in space when it wears out, etc. etc.
Waste vegetation can be turned into fuel fairly easily, too (which is a
kind of solar energy, really). The combination of all these
Earth-based approaches will keep the price of power down way below any
motivation to get power from space.

You're looking at this backward - starting with the assumption that we
will go into space, then trying to justify it