Can you really trust a non-open-source desktop wallet? I am a newbie in the crypto markets, but as a software engineer, it seems to me that a huge security vulnerability could be choosing a wrong wallet.
If you are using a hot wallet, and can't see the app code, doesn't that mean it is theoretically possible for the devs to steal your crypto, even if you can see your private key?
Shouldn't everyone be using exclusively open-sourced wallets for this reason?
Can you really trust a non-open-source desktop wallet?

No

Shouldn't everyone be using exclusively open-sourced wallets for this reason?

Yes, everyone should use open source wallets. And still use small amounts in hot wallets or wallets that are relatively new and experimenting with new features.
More details: What stops a wallet from stealing bitcoins?