Isn’t the “solution” for Sentry that deploying it is such a pain in the ass that no one bothers to really do this?

That Sentry is a pain to deploy is not really intentional, it just happened over the years. However because it's a pain to deploy it also opens up a market for people that create managed deployments so I would say, that if anything, it made it worse. For self deployed Sentry you do not need to pay cent, the license explicitly allows it.