Many of the youtubers I frequent are still good, however for some there is a definite drop in quality as time goes on. Personally, I blame Patreon.
Patreon is a great thing for Youtubers and streamers to get paid for their work outside of nebulous advertising dollars. However, for many streamers I've enjoyed in the past, when they went from hobby they enjoyed doing to potential job/career things started to suffer. Some people can keep up with it, but with patreon money there becomes an expectation of a set schedule and continued dispersion of content rather than doing a video when it felt right to that person.
Again, some people can keep up with it and that seems to be where some channels flourish (Jim Sterling doing a video a week plus intermittent content is a good example), but others are forcing it to please their patreon donations when really they don't have the capacity to keep being interesting.