1) I speak from personal experience and observation. many people go to a subbed video post, they see the link is dead. But they DO NOT search the comments for anyone who provided mirror links. i personally do that and when I recommend people do the same, they go, "are you crazy to go through 687978797 comments? why should i do the same?"
- What's more, most people will just hit reply and say, "Hey there, Subber, thanks for subbing. But the link is dead. Please upload soon."
- When the subber doesn't respond, they don't even check how long the subber has been off line, no longer on lj etc, they keep writing and asking for re-ups.
- therefore, even if you use this solution of sticking up re-up links in the comments of the post itself, 99/100 people will NOT check the comments to see if anyone has done so.

2) moreover, some comms have rules about no re-up of links fullstop, and this includes no re-uping of links in the comments of the post itself.
- in this scenario, your proposed idea wouldn't work either.

3) in some LJ communities/journals, both dead and alive, any reply with a link gets marked as spam and will not appear. this could be the subber's own setting and seems to be LJ's default setting. one example is Hanaraechan's comm.
- in this scenario, your proposed idea won't work because no one will see the re-upped link as it is marked and spam and will be deleted by the LJ powers that be.

4) there are some cases (in other non-arashi comms) where the comm is long dead and the mods have all abandoned ship. in those cases, lj steps in and makes that dead comm an archive. that means, it becomes a comm where no new entries can be posted by anyone to that comm, and no one can make any comments in any of the existing entries in said comm.
- again, your proposed scenario won't work here.

5) what happens if a new fan wants a file from a comm that is now dead and dead since 2009. they cannot join that comm because the mods are no longer around on LJ to allow them into the comm.
- even if you are a member of that dead comm and you put a re-uploaded link in the comments, how will these new fans gain access to that post in the first post?
- even if this dead comm has open membership, what makes you think the new fan will be the 1/100 who will look through all the comments to see if any kind person has put up a re-uped link?
- there's a 99.999 percent chance that even in a dead comm that is still open to anyone as long as they click join, your re-up will be marked and spam and deleted by LJ. even if that doesn't happen, 99 out of 100 people won't go looking at all the comments just to see if anyone has re-upped the file.
- as evidence, i cite you the example of the subbing group T101. That group removed all its download links to its older files in all its comm's entries. what did 99/100 of the users wanting the files now that the links have been removed do? they do not search the t101 comm for the modpost explaining why the T101 team did this. instead, the 99/100 of new users wanting the file, keep writing in the comments, "you claim you subbed this, but there are no download links. why?"
- again, your theory won't work here because that's the way folks are. They are lazy and won't read everything to find out why something is the way it is. they want to be spoon fed.

Another example, the mod account of the Kakenukero subbed archive list is for easy navigation of the archive, but people don't read her FAQ and still complain why her list is on etrangestorm and not in the Kakenukero subbed archive list. they also complain why it's so difficult to find the files they want without a "contents page". If they had only looked at the mod account for Kakenukero subbed archive list on etrangestorm, they will find it. But they do not do that. They also keep complaining that she leaves links to dead comms, deleted comms up even though she has stated on her main page why she doesn't do it.

People just don't READ. there's a 99% chance people WILL NOT want to go through all the pages of a post to look for re-ups.