Thonet vs Eames  

I have a 1960's copy of Arts and Architecture
somewhere where they ran an article addressing the rise knock-offs. Among the many side-by side comparisons of Lightolier vs knock-off, Florence Knoll vs knock-off, Saarinen vs. knock-off, etc.... this Thonet chair was also featured as an Eames knock off.
In the opinion of the industry at the time, it's a copy for sure.
I'll try and find the article.

Molded plywood
As you can see, the DCW has a much more molded form than the Thonet, which makes for a more comfortable and supportive chair. For this reason, I would also presume that it is more laborious and thus more costly to manufacture. Also, I believe the Thonet does not have an additional veneer on top of the plywood.

Product vs. method
The Eames' chairs reflect a method of working that is much different than the Thonet method. Thonet was in the business of producing products - lots of products. Their main innovation - the mass production of steam bending beechwood - was a hundred years behind them at the time of the Eames chair release.
The Eames were designing a way of sitting. The Eames plywood chair isn't simply about making a chair - it's about making the most comfortable chair, with the least amount of material, and for (in theory) the least amount of money.
And the Eames continued to innovate, which drove their popularity and brand recognition. Thonet continued to plug away at making variations on the same thing - with no real dazzling innovations. A similar comparison can be made for Plycraft - who made dozens of plywood chairs, but none were as successful as the Eames (closest might be the Cherner)
And for a true test of why the DCW succeeds, even today, just sit in one. Then sit in a Thonet. The difference is quite dramatic.