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Both Pope St Pius X and Pius XII changed parts of the "cuм Ex" law to allow excommunicated individuals to both elect and be elected pope.  So, to some degree, "cuм Ex" is old news...

1.  Are you really calling a decision by Pope St Pius X "sad"?
2.  Have you ever read his changes?  It doesn't sound like you have.
3.  Do you not understand that changes to conclave (i.e. human law) have nothing to do with infallibility?

Ultimately, the conclave changes made allow an excommunicated cardinal (i.e. which is not the same thing as a non-catholic) to vote and be elected...but...immediately after the conclave, all excommunications, sanctions, etc are re-enforced.

If anyone knew of the dangers of Modernism, and how much rome was infiltrated (already!) in the early 1900s, it was Pope St Pius X.  So, in his papal wisdom, he foresaw that it was VERY probable that future "popes" would be heretics, excommuncated, etc because the infiltration and corruption was so deep.  Thus, it seems he decided that if he could not prevent heretics from gaining office, at least he would (by the conclave changes) allow that the temporal/govt/visible part of the papacy be legit...and so the papacy could continue in some aspect (even if not fully).  We all know the spiritual aspect of the papacy is not legit, as these people have no true spiritual authority.

This lines up 100% with Sede-privationism, Fr Chazal's impoundism, and other hybrid theories.  Don't you see the wisdom in this?

Both Pope St Pius X and Pius XII changed parts of the "cuм Ex" law to allow excommunicated individuals to both elect and be elected pope.  So, to some degree, "cuм Ex" is old news...

If the individuals in question were excommunicated for reasons other than heresy, apostasy, or schism, then that is fine.  However, for the public sins of heresy, apostasy, or schism, no pope has the authority allow such individuals to be papal candidates because these sins by their very nature separate the culprits from the Church.