It's a 3 LCD projector, so if I understand right, light passes through the bulb, into some kind of lens to focus through the LCD layers, then through another lens or set of lenses to focus on the wall.

If the dimming is universal and not limited to one color, then I would expect that it's the bulb, unfortunately.

From what I've read, it appears that not all replacement bulbs are made equal, so it's possible that a less expensive bulb also provides less performance.

If you're not timid about it, you can open the projector to inspect the circuit board for failing capacitors. That could impact the power supply to the lamp. Capacitors are barrel-shaped devices that store energy and release it in a controlled fashion, and if there are any burst, bulged, or leaking caps then they'll be out of spec.