I feel we are actually pretty close to being able to clean up the space debris. My optimism stems from the lowering cost of getting to space, the availability of light weight space avionics, and vacuum usable adhesives. Small, persistent, delta-v changes can be applied to space junk with lasers and from devices which attach themselves to the junk and lower a tether into the magnetosphere and energize it with a solar cell. Further, on orbit refueling on 'cleaner tugs' becomes more interesting with enough xenon gas stations. NASA did one of their challenges about space junk and it had a number of very creative ideas that were predicated on lower cost access to space.

There are real plans to attempt this, at least on some smaller scale.

Astroscale is one example: http://astroscale.com/

At some point it would be possible for an entity to either launch (or more likely refine in space) enough materials to focus sunlight across not /quite/ the earth, but the orbital paths. Since 'light pressure' exists, this could be used to counter existing acceleration and thus encourage orbital decay.

It would take a while, it would be costly to start up, and boy does this sound like a weapon from 50% of space sifi stuff I've ever read or seen...

However I suspect it's still the cheapest solution to the issue.

If it can push a satellite down from a great distance, and likely seriously heat it up and damage at a shorter distance, it is a weapon, the same way a knife or an axe is a weapon.

It's the way you use something that makes it a weapon.

Things are what they are, not what our arbitrary categories classify them. A knife is a tool and a weapon at the same time, and it can be a great number of other things, a cultural artifact, a piece of industrial design and a piece of junk, all at the same time. A thermonuclear warhead device can be a doomsday device and an essential tool for Mars terraformation.

So it's not the way you use it that makes a weapon, a weapon remains a weapon throughout it's non-violent uses; the oldest trick in war is to disguise weapons as tools before using them in an attack. See nuclear proliferation under the guise of atomic energy, the militarization of space, Hitler's powerful 'defensive' army.

It may sound like a pedantic distinction, but in practice, the fact that something IS a weapon, regardless of how it's being used, will drastically alter decisions. Imagine an unknown person coming at your family with a knife or ax in hand. Under the "not a weapon until used as such" doctrine, the burden is on you to prove the agresive intentions and you have no logical reason to be concerned until then.

I wonder if it's cheaper to fix it later or prevent it now. There might be huge opportunity cost today if we obsess over garbage collection prematurely.

Vastly cheaper to postpone now just don't send junk up, but the problem is mostly limited to keeping satellites in space not simply leaving earth.

Even then in LEO you get enough drag to limit risks and the worst case ends up as a trade-off of safety vs continuous fuel usage. Remember, space is vast but satellites and space junk are both really fast and stay up for a long time. Prevent junk from staying in your orbit and the risks dramatically decrease even more so if you add maneuverability. Even better LEO prevents your satellite from adding to the mess long term.