I don't think anyone is expecting ALL of the old warcraft3 modders to come back; I'm sure most have moved on from mods like you say. Regardless, those guys were probably making maps for fun when they were 15-25 and that's potentially what sparked their game dev career. So who's to say a new generation of 15-25 year-olds or passionate modders can't create some original awesome maps or update the old classics?
Kind of covered this in my other reply, but I'll go into it a bit more.
Successful modding, in the WC3/Dota 2 sense, requires some amount of technical ability and game design sense. This combination of skills makes you uniquely suited for a designer or technical designer position at a lot of studios (which, for a lot of modders, was what they aspire to be when they start modding).
When Warcraft 3 was in its heyday, indie studios weren't really a thing. Physical distribution was the norm. The Eastern and Western markets were completely separate. The only product form was the $50-60 boxed product. F2P was unheard of in the West. Tools were limited.
People mod for a few reasons--as a creative outlet, for the experience, because they love the product, etc. The idea of being a professional game designer was out of reach; game design wasn't even a well-defined role at many studios, and there was no widely accepted career path towards being a game designer (in a lot of ways, there still isn't). That's one of the major reasons mod output in the 90's and early 2000's was so high. Now we have Steam, Kickstarter, digital delivery, various prototyping and game making tools, and a much clearer definition of what the game design role is. It's become much more acceptable and accessible overall--so a talented designer is much less likely to use modding as an avenue for creative expression. It's like telling an amateur musician that 1. you can actually sell your money on iTunes now, and 2. there are schools for music! Will there still be people who put their music out for free? Of course. There's also a lot of professionals who mod now for the *potential* of profit (see Valve's workshop).
So again, it has nothing to do with "the only good modders were WC3 modders". It has to do with the different opportunities available to the current generation of talent.