I still have some doubts about the issue: they're not releasing new series or ceasing the production completely?
As far as I know, there's no difference. My understanding is that each new series runs for a while and then stops--there are no "evergreen" DDMs. WotC appears to be getting out of the minis biz.
What I mean by "alternative" is another company that produces minis, similar in quality and price.
Well, yeah, but what's your definition of "quality?" And, for that matter, what's your definition of "produces?"
I mean, Reaper metal minis have excellent quality in terms of sculpt and detail, far better than DDM, and the selection is top-notch. However, the paint jobs are... well... lacking (as in, nonexistent) and the prices are high. Likewise, they score lower on durability; you can't just throw a bunch of Reaper metals in a bag and cart them along to game night. So whether Reaper metals are a good option depends on what you're looking for. The Legendary Encounters stuff meets a different need, closer to DDM in terms of manufacturing priorities, but with a smaller selection and higher prices since they aren't randomized.
And for "produces"--do you care whether the company makes the minis itself, or just whether it sells them? You can go to RPGLocker, Auggies, or Troll and Toad, and get a heap of DDMs at excellent prices. If you're looking to build up a good collection for gaming purposes, I doubt you'll find better. On the other hand, if you have a big collection already and are looking for
new minis, obviously the old stuff isn't going to appeal to you.
If having new minis is important to you, Legendary Encounters may be your best bet.