living longer isn't ASB. 20 years is pushing it, but not impossible.
without more offspring, Spain remains more stable as long as he's capable. Once he's invalid, or dead, the same issues plague Spain as OTL. Aside from Portugal, which may cause butterflies, not much difference.
IF he lives long enough to pump out more kids, a lot changes. He lived 4 years after Carlos II was born, but no more kids. Toward the end of his life, it was becoming obvious that CII was severely handicapped. Maybe Philip climbs into the saddle to produce more heirs. Hereditary issues are a concern, but I don't think a second bewitched one is automatic. A son absolutely is a game changer. He might jump CII in line of succession (Carlos III's eldest son was bumped for being infirm), but even if not, he succeeds CII. For butterflies, see one of the many "WI CII had offspring?" threads. Same effect. A daughter (no more sons) would be interesting. She would be in line for the throne, as no salic law, yet. She probably has a harder time jumping CII, but she might be kept around as a spare. Certainly, her marriage is going to be of world interest. I wonder who it would be. The only Bourbon, or Habsburg I could find around the same age was Louis, Grand Dauphin, and that isn't happening. Some no name/lesser house who would come to Spain as consort?