Isn't the full modlog stored somewhere offline?
I think that the problem is the resources to make a search in a giant modlog with years of entries, more than the size itself.
And in some decades the size may be a problem too if many people decide to download it(bandwidth problem instead of storage problem)
No, it's basic unix/linux sysadmin shit. you log stuff then archive them off weekly/daily/monthly as needed. I personally keep a year's around, archived monthly (not here, my own sites).