If it's currently available, it's probably not worth the bother to do another version unless you feel the existing one isn't well done.
I've noticed a few series that were put out by publishers that are no longer around have been put on MangaDex, which seems reasonable.
The cheap paper used for a lot of manga and comics makes digital copies a lot more attractive. There are steps you can take to make paper last longer, but it's harder when the stock isn't as good.
The best way to preserve anything is to follow the 3/2/1 method (proposed by a photographer, sensible for any media):
3: At least 3 copies (as a minimum, not an absolute)
2: On at least 2 media (paper, hard drive, cloud, flash, optical, et cetera)
1: At least 1 copy offsite (so a disaster at home, or at the office, and so on, doesn't destroy your only copy)
As online storage and portable media have become cheaper, it's gotten a lot easier to keep more copies of things around. MangaDex is one of the current solutions, keeping in mind that it came about as a replacement for another site shutting down. One of anything is not backed up, by definition.