IMO i think they should follow the way of iTunes match.
You should be able to buy the physical copy, then be able to match it up on their servers (comixology) to be able to have the digital copy as well. And I'd gladly pay 25 bucks a year for this little option, ONLY if it's just like iTunes Match in the sense that i can actually download the digital copy and keep it personally instead of making it a propriety file that can only be opened with their app or program (this way i can save it on a backup hard drive of thumb drive if i wanted to).
if they made it to where it's basically a pdf file, but with special coding in the background to where when it's opened with their app you can do the nice panel by panel viewing, that would be perfect. that way, if comixology ever goes down, you still have the pdf files of the comic that can be read normally by just zooming in and moving around with your mouse (or finger if on a tablet or mobile device).
and if comixology ever did go down, you know there would be some knockoff programs for free that would imitate the same panel by panel viewing feature that comixology has going for them so you can view those pdf files with those programs.
This is how i enjoy my music. I like buying the physical copy for sentimental sake and for personal reasons. but i also enjoy the pleasure of having digital copies that can be available at any moment when i need them. i usually keep my latest 6 cd's in my car to listen to, but on the rare occasion when i want to listen to something old, i can always pull out my iphone and pull it up in a flash. and it's a nice digital backup of all my music just in case something would ever happen to my actual CD's (fire, stole, lost, etc...) or just a good backup just in case something happened to my laptop or hard drive that was storing the actual digital backup's of the music. if my hard drive broke and i lost all my backup's, it's ok, cause itunes has all my music remembered on their server so i can just download them right back onto my laptop then do whatever i want with them.
I think this is the most sensible way to go. it would appeal to everyone. iTunes has been dealing with how to properly go around this whole physical/digital copy of media for a while and i think they've figured it out in a way. and i know comics are new to this, so they should take a look at other media platforms and take note from them; see what's worked and what hasn't