Fission is also massively expensive and a typical
reactor can take 15 or 20 years to be built.

Bureaucracy.

And a dramatic increase in fission will require a solution
to the nuclear waste issue that has yet to be solved.

It is solved. We are talking about few thousands cubic meters.

But I suppose we can turn Nasa into a great big
Waste Management company, and have them
blast the nuclear waste into the sun~

No comment.

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.

I think it's rather pointless to do anything other than speculate about
next century power sources. If you look up the energy situation during
the early 1900s it becomes pretty obvious.
Even so, IMHO in 100 years fusion will form the mainstay of power
generation.

The future should define the present.

I always thought that it was the other way around.

Consider this:
-current energy consumption is approx. 450 Quadrillion BTU (4.5 x
10^17) = 1.31882 x 10^17 Wh = approx. 15x10^12Wh installed power.
-current Si solar pannel weight is around 50W / Kg (GaAs pyramidal 150W
/ kg - probably too expensive).
-projected growth of energy consumption at aprox. 10% per decade.

Asuming same consumption, total installation (that's trusses and
antenae included) efficiency at 100W / Kg !!! a launch cost of $250 /Kg
to GEO and transmission efficiency of 80% (we are talking 'bout the
future heah), you would need ... mmm ... quiet a lot of dinero to
replace ALL current sources.

Now, how big a slice of the energy pie you want to cover with solar
CLEAN, WONDERFUL power ?