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DVC announced 2018 extended points can be reallocated to existing reservations that occur before Nov 30, 2020, then borrowing would not be that risky if you have, or plan to have, another reservation before the end of your 2020 UY.
For example if an Aug UY borrowed points to make a reservation prior to November 30, 2020, and also had, or planned to have, an existing reservation sometime in 2021 before July 31st, which used 2020 UY points, then they could replace those borrowed points with the extended 2018 points when the extended points become available. After that they could use the freed up borrowed points to replace the 2020 UY points in their prior to July 31st 2021 existing reservation, and then bank the freed up 2020 UY points. The only downside is you would lose the ability to bank the 2021 UY points you borrowed into your 2022 UY.
The problem is that DVC has not announced whether extended 2018 banked points can be used for exiting reservations, and by the time they do availability prior to Nov 30, 2020 will probably be almost zero. Given that lack of announcement I agree with you when you said: "Someone would not want to borrow"