I think if Melton is healthy, he'll actually be a better player than Hatcher was. Or at least better than Hatcher was likely to be this coming (non-contract) season.
However, overall, you are right. This DL is at least a 2 offseason rebuild, and let's not forget the OL needs attention still too, it's not done just because we've locked down C and LT. The guards are still undertalented and RT is a crapshoot -- Free is on his best days average and his worst he's a catastrophe.
Anyway, the DL just can't be fixed in one offseason. We are essentially having to turn over all four starters from the projected 2013 lineup of Ware-Ratliff-Hatcher-Spencer, and that group itself had such suspect depth that addressing it was already a priority. I mean, even if you count George Selvie as a keeper (and I'm ok with that as long as it's depth), you probably need 7 more names on the DL.
If Crawford is one, and Melton is one, we're still at 5. Hell, even if Ben Bass is one, you need 4 more. You can't get them all in the draft, and we have no money left in free agency, and Crawford and Bass are looking to be the definition of multi-position tweener wave players who definitely aren't starting material. And no, I wouldn't count re-signing Spencer to a one year stopgap contract as fixing the situation.
It's depressing knowing that there is really no way this DL is gonna be patched together to allow this defense to be anything more than painfully lower-mediocre next season. Even if you devoted 2-3 draft picks to it, it's probably still not a strength unless they are all home runs. And you do that at the expense of getting serious and much needed OL and S options on board.