I'm building a SaaS service, very early stages (rough MVP) and planning to set up a sandbox.mysaas.com where users can play with the service under a preset "Test User" account without registration.
Is it a good idea from a business perspective or am I just missing on potential customers' emails?
Here is what I will do. Give them access to a sandbox but let them create their own userid for it if possible using their email etc. This way, you now know how many actual users are trying your app.
This will also reduce the number of non-serious users as well. Yes this may cause some users to not try the app but in my experience, any user who does not give you at least their email to try your app is very unlikely to become a paying customer.