Are you seriously suggesting a 20 year sentence for benefits fraud?
Ignoring the fact that such a system would be ripe for abuse and retaliatory behavior, just accidentally convicting an innocent person would absolutely ruin their life.
Getting wrongly convicted is already a huge tragedy. Now you'd be handing out life sentences basically.
Let's be real, it's not the rich people who tend to engage in benefits fraud and I'd argue most people who do this are in desperate situations.