I've been doing WP tech support for over 10 years for themes and plugins. I agree that most of the time issues can be resolved without a need to access the client's admin. However, depending on the issue, sometimes it's impossible to diagnose what's going on without accessing the backend.
In those cases, not having access to the admin is like trying to fix a car without being able to look under the hood.

In my experience, most customers have no problem providing credentials but in those rare cases where the customer cannot provide admin credentials for any reason, I would put the ball on their court. The customer needs to provide a place (staging site) where:
1. the issue can be reproduced
2. support staff can access
3. there's no sensitive data

Customers also need to be educated that their site is their responsibility. I often hear the "I am a (insert non-technical profession here) and I don't have time to learn about FTP (or any other basic website tool)". Guess what? Yes, you do. Depending on the type of site you run, maintaining it can be a full-time job.
If you rely on your site for income in one way or another, you need to be able to perform basic to intermediate troubleshooting procedures and not rely on plugin/theme support staff to fix your issues.