You still get plain text if someone in your garden tries to communicate with someone outside of your garden. Or, alternatively the folks outside your garden get an awkward link to your website where they can only view the message there.
I don't think those are very good reasons for doing what you did, since the set of users not using protonmail is orders of magnitude larger than the set of users that do, so protonmail is basically just covering a very small edge case (communication between protonmail customers) while making it more difficult to use the more common case of communication with folks that do not use protonmail.
It's not a small use case for corporate users where the internal mail is all encrypted. We also have full PGP support and the bridge is fully integrated with this, so we hope the garden aspect will decrease with time, though we expect PGP volume to remain small. You are also heavily discounting the overhead of having users manage their own keys locally in your preferred solution.
I don't think those are very good reasons for doing what you did, since the set of users not using protonmail is orders of magnitude larger than the set of users that do, so protonmail is basically just covering a very small edge case (communication between protonmail customers) while making it more difficult to use the more common case of communication with folks that do not use protonmail.