1.- > "But nobody wants that." Do you know everyone's thoughts? Have you interviewed everyone?
First: That was a double-meaning from me: All People who answered to you here in the thread, but also "all".
I don't know everyone, but i know it for my known people and companys, thousands and representative enough for me.
(perhaps more located in Europe, but exactly this is my professional World) I would think I can appreciate that very well.
Also i read online about other people all over the world and all what i hear is the same, the exceptions are very special or very small.
Protecting a drawing like your older way (proxys) are not new and it's enough for many user,
this works well (without additional software on the other side) and wasn't the game here&now.
>" but perhaps there are not many other cars, vans, trucks ...?"
Yes, a mass! The world is extremely colorful, BUT the mass of people uses just one color, or 4, or 6 and not 16,7mio/endless
There are only two Variants:
a) People who like to protect there cad data if they give these out of hand to all potential dealers and customers
b) 1. Exclusively clients, who like to protect there cad data while working in a small, limited environment (one project, or one special customer)
and and which may contractually dictate the use of a particular software. Beyond that, you also want to accept these hurdles.
And without any evidence for that, I say: 99.9% of all belong to a)
The group b) can also include many thousands, but these are very few in relation to all and thus rather the rare (or very small) exceptions.
In addition, I expect many problems with your solution side by side with other applications,
but I could not test - did not get it running as i wrote above (and then lost the desire).
2.- >"..."
I totally agree with Rob&Alfred..
>"On the other hand you also need to read the messages clearly"
I did this and i know that and you should also notice that this user did not reply to you in the thread and that you subsequently only talked to us.
AND: You should do the same (read the messages clearly) and if you had done this, you had stopped the work at exact the very first thread-open post:
3 years ago, @Anonymous wrote:
"I've been given one once before but I cannot seem to recreate the process. All of the third party applications I've come across"
"seem to be pretty convoluted when it comes to the recipients process for viewing the file."
@ this point i know, Rseybert is part of Group a) and you're way is 100% the opposite of what he searched for.
It sounds he is/was search the same what we also talk about (but we know, it is not possible).
>"But since Today on I will promote my work in other places."
The poor people (just kidding) Then we'll read about you in other places if it happens publicly.
Good luck, you will find the small exceptions (customers), for sure.
But as I said, the solution via 3.part program to open a file) is simply uninteresting, unattractive, impractical for the general public.
3. - .
4.- My horse is NOT dead.
Of course you think that, otherwise you would be stupid and not have developed this version and would not even try to sell it.
But as I and others agree, the path is not successful when it comes to reaching many users as customers.
Q: Why do you think that there is no big widespread established solution on market that goes exactly the same way as you (3.part program to open the file)
Because you're the first one to have this brilliant idea?? 
I hope you understood most of my answers.