This article surprises me as a commercial theme/plugin seller but not as customer myself...

We have a higher plan that includes "we'll log in and fix it if you can't after following our instructions". It is a matter of last resort that makes things easy for a customer in a pickle, especially someone new to WordPress. Never once has a customer rejected our offer to log in and help and we've destroyed exactly zero sites.

You have to do it right, though:

Have a very clear Support Policy and never stray outside of it
Ask the customer to make a full backup before providing access
Ask them to create a new account rather then sending their own
Get WordPress admin access via the new account / password reset process
For FTP, instruct a secure means of transmission (never email)
Store credentials securely (e.g. password manager) and only temporarily

With that said, I personally have not and never would give access to my own critical websites to a third-party, even a reputable one, because I do not trust anyone either. This my livelihood. I trust myself with other peoples' sites but not other people with mine.