It's okay, we'll all be dead before they all die, making our goals perfectly safe to obtain them all!

Well hey!  That's good news, but  you're younger than me so you've got longer time left on this earth to play these carts.  But if they do go out in my lifetime... you'll be hearing from me.

Just wondering if n64 cartridges would be the same deal? (i.e. as nes/snes.

Also, and most importantly what I want to know, are 'not for resale' n64 cartridges made the same way as normal n64 cartridges? (and so just as resistant to corruption).

Yep, they're the same. AFAIK, N64 demos are exactly like retail carts ROM-wise.