What's even the point of comparing global WeChall scores, beyond getting a general sense of how active someone is in this type of thing?
Some people only like a certain type of challenges and not others, meaning some really skilled/knowledgeable individuals might have way lower global score than others just because they lack as much "breadth" in terms of topics covered.
Besides, you can also just do easier challenges on a wide range of sites but never complete one.
You may end up ahead of someone with the same "breadth" as you but more skilled/knowledgeable than you who focuses on a smaller number of sites and takes their time to tackle all challenges, including the really hard ones needed to get to 100% on those sites.
What should be rewarded more, breadth or depth?
So while I do agree there is some correlation between skill/knowledge and high global scores (especially the really high ones), it's generally not a good metric for comparing yourself against others.
Comparison on a site-by-site basis is what really matters from a competitive standpoint.
I do agree that some kind of filtering in the leaderboards of all/only live/only dead sites would be a "nice to have".
As for point decay, I am against it because I think it will create unnecessary FOMO.
If you want more "time pressure", play a CTF, other sites already have leaderboards for that.
WeChall scoring currently already breaks ties by date, which is a good way to encourage people to solve challenges sooner rather than later, without creating the stress of losing points if you don't.
Personally I wish there was some sort of preservation project for these kinds of sites, like what's happening with TBS.
Smaller gestures and efforts also count, for example at some point before I managed to solve Electrica, its domain expired so I e-mailed Caesum and soon after it was back up.
IMO it's better to keep the scoring around optimistically, because you never know when a site might be fixed/revived so you might end up deeply regretting the deletion of those scoring records.
Plus keeping the scores for dead sites functions as a historical record of your progress/journey, which is valuable in itself.