"First you say "astronomers have found a way to get a clean view of the disks surrounding black holes by using a polarizing filter in the infrared."
Then you say "Now, next on the agenda should be developing a suitable gravitational wave detector to confirm the existence of black holes!"
If you can see the disk around the black hole, doesn't that confirm the existence of the thing?
I guess not…"

>>>I guess it's all about levels of surety. We are 99% sure that black holes exist, based on numerous observations. If we observe them directly via gravitational waves, then we are observing the actual BH itself, and hence our confidence grows even further. If we have multiple different lines of solid evidence, it bolsters the case to a point where it really is, for all intents and purposes, inarguable to disagree with the conclusions.

A paraphrase from 'Stephen Hawking's Universe' sums it up fairly well – 'A clever theorist can come up with any number of explanations for a given observation, but it becomes ridiculous to argue the contrary if many independent lines of evidence lead simply to the one conclusion, and many different observations are neatly explained by one theory.'