You're contradicting yourself. In this post and others you give a Sedeprivationist view, saying that the heretic Pope is implicitly impaired. But then later on you said that the Popes abrogated Cum Ex SO THAT a heretic could be validly elected Pope, to make sure the Papal See remained occupied.
It's not a contradiction, if you distinguish between the material/govt and spiritual office.
Why would they arrange it so that the Papal See would be occupied by someone who would then by spiritually impaired from their papal duties?
I can't answer for them, but I have some guesses:
1. The material/govt office is important because it is the VISIBLE sign of a papacy, which is a visible sign of authority and unity of Catholics. To many catholics, especially in these times of crisis, having a leader (even if it's a bad leader) is important to their human psyche and morale. It helps to keep unity and order.
2. If the material/govt office remains occupied, then also the Church's govt continues to operate, which means dioceses, parishes and schools keep operating and bishops who die are replaced with new bishops, etc. Also, the jurisdiction of the Church continues, in a temporal sense.
The material/govt office is very important, just not as important as the spiritual. By changing church conclave law to allow a potential heretic to be elected, you keep the Church operating and you keep up appearances, you "keep the lights on". Once God intervenes to have mercy on the world by ending this crisis and blessing us with the resurrection of His Church, and a return to orthodoxy, then the new hierarchy can hit the ground running and "clean house" very quickly. Odds are, if an orthodox pope were elected, there would be a good % of novus ordo bishops/priests who would convert also (and some of the Cardinals and hierarchy too).
If the conclave laws weren't changed and we abided by a strict interpretation of Cum Ex, then basically, there's no hierarchy and 95% of bishops/priests have lost their office. There would be an end to any continuity of the Church govt. Any future orthodox pope would have to "start from scratch" in the sense that most dioceses would be without any authority, structure or governance. It would be the wild west for a few years (at least) until the Church could fill all the open positions and teach/educate these new bishops/priests how to operate a diocese.
I think the decision to allow a spiritually impaired pope to exist is a practical step which is prudent in the times we live, since both Pope St Pius X and Pius XII saw the growing Modernism and envisioned a future where Modernism would corrupt almost everyone (and they were correct). Practically speaking, a bad pope is the same as no pope, because the spirtual guidance and leadership does not exist in either case, but on a temporal level, there are some benefits to having a visible leader continue the human operations of the Church.