Comment by pdpi

The fundamental problem with the "Why didn't you just <solution>?" format is that implies the other person knows less than you. Going down the self-effacing route just goes from "I think I have the answer" to "even though I don't know a lot about this, I still think I have the answer". It's actually worse.

Instead, I prefer approaching the discussion as me catching up to the current state of affairs. "What options have you looked at? Why haven't they worked?" followed by "Ok, have you considered/tried <my solution>?" Both fixes the subtext problem while actually giving you more information to work with — you can now choose to offer the "obvious answer" or not depending on their response, and potentially have a stronger argument for why it's a good fit and/or some caveats about why it might not be a great fit (but still worth trying)