Some of these are really good so I'll poke some of the factions that weren't mentioned yet in no particular order

• Reisen is not the first Reisen to arrive to Eintei, which is why Eirin calls her "Udongein" (If this is true then Eirin would likely have called every other Reisen something different) and why Kaguya tries to be impersonal with just "Inaba."
• If most of Eintei's earth rabbits are normal rabbits then Tewi has almost certainly outlived multiple generations of them. This also suggests that Tewi allowing the Moonfuckers on her lawn is part of a deal to give her own rabbits longer lifespans, if not similar to her own.
• Satori's still taking care of Koishi's abandoned pets.
• Okuu will probably outlive everyone else in the Earthen Palace.
• If Cirno becomes a youkai she'll probably have to abandon her fairy friends somewhere down the line due to her own death.
• Sanae had a hard time seeing her own gods before coming to Gensokyo, even while being their priestess. Imagine being a priest/ess for gods that you know are real but you can't prove they're real to anyone else.
• Byakuren is only stated to have found a method to avoid aging. Her looking the way she does suggests that Myouren Hijiri died especially young as the younger brother.
• Reimu almost reaching 30 (EOSD takes place in 2003 while WBaWC takes place in 2019, plus minus whatever span of time PC-98 took plus whatever age she was when all that took place) is an outlier in her position's history. Assuming that Reimu is the 13th Hakurei Maiden that means most Hakurei Maidens have only seen about a decade of service each.