Payware models exist for two reasons:
1. The Auther needs money to pay some common expenses [bills, cost of distribution, etc.].
or
2. The Auther is just plain greedy.
I have already released the Mercedes-Benz SL600 for FS2004 [as you already know]. And it's freeware.
I made it freeware because I was new to the field of "car-modeling" for FS. Also because I didn't have a bank account for on-line purchases from customers, and I am not greedy like a few people in the Payware Market.
Project Mercedes has greatly expanded my skills and knowledge for car-modeling in Gmax for FS.
Now that Project Porsche has begun [Porsche 911 GT2], my new skills and knowledge will make this new car project a better one.
The Porsche will still be "Freeware" as it is intended to be, along with new and improved features. I'm a Freeware freak.

It is very "UNLIKELY" that this new car will be "Payware" as well. Even if it did go payware, I would have priced it at no more than $2 and it would have eventually become freeware [as the case of the Golden Eagles' Cubbie].
However, I do know that making "payware" models is a very delicate operation and a very serious one. Especially for a person like me who will forever make detailed car models for FS.
PS: My Mercedes may not have a 2D panel as it was supposed to, but at least it has a DVC and other stuff.