A 5% grade will be significant drop in speed, well over 50%.
Bicycle Speed (Velocity) And Power Calculator
A 180lbs rider with 20lbs bike on the flats and 200W goes 18.4mph. A 5% slope drops that to 8.4 mph (200 lbs cyclist is 7.7mph). This is all assuming optimal gearing.

At the same time, dropping the rider power to 150W drops the speed to 16.5mph. Which is where most people cruise at solo. A sustained 200W is a hard effort for a fit recreational cyclist.

You're right that if 200W is comfortable, it'll also be comfortable climbing. The trick is that most people aren't comfortable at 200W on flats. They're actually comfortable at ~140W on the flats, and so bumping it up to 200W for the climbs is a significant effort increase. Very few people climb at ~140W, because it's dead slow (6mph).

As someone who climbs at ~200W, I can tell when I'm doing a similar effort on flats, but it's much easier to dial back a little without loosing much speed.