but if I recall correctly, the descriptor itself in some cases contains an extended private key - if not, then it's a watch-only wallet - and in such cases, you can simply use another software to calculate the derivation path in accordance to the descriptor wallet, which will then yield the private keys.
There's a reply three post above yours that shows how's it done.
New Bitcoin Core wallets contain descriptors with the master private key, the only cases it isn't is if the watch-only wallet is specifically imported with descriptors with extended public key or single key descriptors.