First of all, accidentally replying in plaintext is hardly the only problem with PGP, just the most obvious one. Secondly, it's not user error: modern messaging cryptography is designed not to allow it to happen.

Modern cryptography should also not allow users to activate a sketchy linked device feature by scanning a QR code:

"Because linking an additional device typically requires scanning a quick-response (QR) code, threat actors have resorted to crafting malicious QR codes that, when scanned, will link a victim's account to an actor-controlled Signal instance."

This is a complete failure of the cryptosystem, worse than the issue of responding in plaintext. You can at least design an email client that simply refuses to send plaintext messages because PGP is modular.