[where this model breaks down] -- Alice and Bob go to your website and have a conversation. Eve hacks into the website and modifies the E2EE code. She can switch between serving the normal webapp and the malicious non-E2EE webapp. There's no good way to detect it. There are people out there who really like end to end security, but don't like browser-based e2ee because it doesn't have end to end security.
More generally: since you have to trust the server, you might as well just rely on TLS, because the "E2E" extra stuff isn't actually buying you anything. This is the Telegram model.
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[where this model breaks down] -- Alice and Bob go to your website and have a conversation. Eve hacks into the website and modifies the E2EE code. She can switch between serving the normal webapp and the malicious non-E2EE webapp. There's no good way to detect it. There are people out there who really like end to end security, but don't like browser-based e2ee because it doesn't have end to end security.
Note: https://www.cyph.com/ is a bbE2EE chat system.
More generally: since you have to trust the server, you might as well just rely on TLS, because the "E2E" extra stuff isn't actually buying you anything. This is the Telegram model.