Your last question first: no it won't always consume 100 W, that seems to be a common misunderstanding. A 100 W power supply will supply 1 W if that's what the load needs. It's what you attach to the power supply which determines the actual power consumption. The 100 W is just the limit.
Bench power supplies are a possibility, but they are too sophisticated for this: they are designed to deliver 1 or more variable voltages, while you can do with fixed voltage levels. Also, a bench power supply usually has only a couple of voltage outputs, so it may depend on your needs if that's sufficient. You already mention three different voltages, but you might also add 5V DC, maybe others.
The lack of a standard makes it hard to design a universal supply; there's not even agreement on a standard polarization for the DC connector. So each power supply is designed for its own purpose: a given voltage at a given current. Today many adapters use switch-mode regulators for more efficiency, but also these are optimized for a certain output voltage. You could go from 15 V to 12 V to 9 V, but each level has its efficiency, and for the last link in the chain you would have to multiply all previous efficiencies, and the 15 V will need to be higher power than when it just has to power that router.
What you could do is remove the electronics from the adapters and put them all in a single enclosure, that would at least save you a couple of wall sockets. But you'll lose the possibility to wire every device up where you want; do you want to place your speakers in the same closet as your router?
Final note: a PC power supply delivers different DC voltages, but most power is available at the lower 5 V, maybe not enough at 12 V, and at tens of amperes at 5 V it's way overkill.