Draft 2 Digital doesn't distribute to Amazon or Google Play, as far as I know, but they cover a lot of other places. Many self-publishers who go 'wide' (meaning, not Amazon exclusive) use D2D. I'll definitely use them if I ever decide Amazon exclusivity isn't for me.
However, keep in mind that D2D takes 10% ON TOP of the royalty cut that the various marketplaces such as Apple ibooks already take. Decide for yourself if the convenience of having one company handle all your marketplaces is worth the royalty cut.
(Note: after years of studying this stuff, I'm a walking talking self-publishing encyclopedia. Ask me anything!)
I don't believe in traditional publishing. Maybe I'm jaded after hearing too many horror stories from authors who published through small presses (and even some Reddit horror stories from authors who got a big 5 publisher), but when your web serial already has an audience, there's little to nothing a publishing house can do for you that you can't do (possibly better) yourself. Sure, they provide editing and a cover, but you give up most of your royalties and your control. Contrary to popular belief, publishing houses don't necessarily make smart decisions in regards to pricing and marketing, and I hear the quality of editing is slipping of late, too. They save costs wherever they can, often to the disadvantage of an unknown new author. Don't expect a publisher to 'push' you when you're new. They spend their marketing budgets on their bestselling authors.
There are many publishers out there just waiting to snatch up a popular web serial with a huge audience such as Worm, but the terms they offer are usually 90% in favor of the publisher and to the disadvantage of the author. Wildbow is smart to turn those offers down.
Regarding Radish: Looking at the huge amount of negative reviews the apps receive, I doubt it will ever take off as a platform for web serialists. The prices are set in a way that you pay per chapter, and if you keep paying for each new chapter it will cost something like 20-30 bucks to finish a novel length serial. Readers are smart, they notice when they're being ripped off. Many of the reviews say 'use Wattpad instead!' There doesn't seem to be a way to 'save' where you stopped reading, and no way to read without active internet connection. There's no reason for readers to use Radish except maybe to follow their favorite author who is Radish exclusive, but will they keep following that author when they notice the insane cost? The quality of the stories doesn't appear to be better than the quality of popular 'FREE' stories elsewhere. Besides, they're strongly romance focused.
Edit to add: Tintenteufel's situation as a German language writer is unique and I agree that in his case, trad publishing might be the better deal. Self-publishing to the German market is waaaaay more difficult than doing the same in English.