Delegation made it possible for non-invested users to rent stake and run vote-selling bots practically risk- and accountability-free

This could already be done and was already being done with vote bots where users signed up with their posting authority. Delegating your SP to a vote bot (in exchange for pay) and giving a vote bot the ability to vote with your own account (in exchange for pay) are essentially identical. It was already happening on a moderate scale prior to delegation and what changed things (including the scale) between then and now was not so much delegation, but the passage of three years of time with more and more people recognizing value of taking maximum advantage of the incentives offered, and then doing so. That was and is inevitable given broken incentives with or without delegation.

Delegation was never at the core of the problem, though it may have contributed in a small way.

But realistically if we want to improve things now (years late I think we will both agree), we need not relitagate the mistakes of the past, however we might personally rank their severity.