I didn't say anything about them "storing messages in plain text". I said that they don't do E2EE by default and since they have the keys for the TLS that encrypts data in transit, they can read the content of your messages. Encrypting their drives - something that any decent service does - only protects you if someone "steals" a drive: Telegram has the keys and can obviously read the contents of their drives.
I found this Kaspersky blog post which provides a nice tl;dr. They even make the same point as me:
Let’s go straight to the root of the problem: Telegram is a unique messenger with two types of chats: regular and secret. Regular chats are not end-to-end encrypted. Only secret ones are.
No other messenger does this: even the notorious WhatsApp, part of Mark Zuckerberg’s data-hungry empire, uses end-to-end encryption by default. The user doesn’t need to do anything at all, there are no special checkboxes or anything: messages are protected from all outsiders (including the service owners) right out of the box.
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This is not new. Back in 2015, Edward Snowden had this to say about Telegram's defaults:
I respect @durov, but Ptacek is right: @telegram's defaults are dangerous. Without a major update, it's unsafe. [source]
To be clear, what matters is that the plaintext of messages is accessible to the server (or service provider), not whether it's "stored." [source]
In practice, they're no different from Messenger, Slack, Discord or a direct message on Reddit. Most messages on Telegram can be read by them, just like Google can read all messages in your Gmail.
Why is Signal or WhatsApp better? Because they do E2EE for all messages. It doesn't matter if they forget to encrypt their servers, all they see and store is encrypted messages. You hold the keys, not them.