Can standard cartridge wheel bearings "shrink" over time? My front wheel has started to wobble a bit at the hub and it's most noticeable when braking at the lights, as the front wheel is doing like a "pulsing" sound when braking - like as if the wheel is off center.
I've pulled the hub apart, but I cannot see any obvious issues, yet when I lift the front wheel off the ground, as it's attached to the fork, the wheel slightly shakes when I tap it. It is not firm as it should be.

Because I cannot see obviously what's going on, could the bearings inside the sealed-cartridge have worn down in size? There is ample grease on them, but I guess even over time wheel bearings do wear down a micro-millimeter or two? That's the only explanation I have as to why the wheel hub has a slight wobble / shake.

Wheel is Campagnolo Vento-Reaction, about 5 years old now.

Here's a video of the wheel, showing how to get to the hub bearings...
The wheel in question uses sealed bearings. They are generally not serviceable. Service consists of replacement. You mentioned they appear to have plenty of grease on them. With a good bearing there should be grease in them and sealed in place by the seal. The only time grease will be visible is if the seal has failed. The seal usually fails because of bearing wear allowing movement.