Right now they're not just bringing back old mons, they're using the x-antibody to also give us new content. That's great. If it makes a year old digi obsolete, big deal. It's new content and it's useful.
THAT'S POWER CREEP. LOOK IT UP IF YOU DON'T GET IT BY NOW. Again, I think a case can be made for older dm mons, but if they're just going back to old mons that aren't getting re-released through a gatcha or an event, then don't bother. Give us something new instead to take the place.
When I say new mons, I don't mean something we've never seen before in DH, they can take mons we already have and make higher rarity cards of them, use different artwork, give it different stats/skills/rarity/race whatever, there are lots of ways to switch it up.