This argument never holds up. Being a doctor or an engineer is almost always thought of as a good path. But what if everyone was a doctor or engineer? There'd be no one left to do basic tasks in life and the positions available for doctors and engineers would be so in demand that these people would work for very little.

Your point can be rephrased as, "What if everyone ignored the realities of the job market?" There's no danger that everyone will do that, so there's no need to use ethical or other philosophical arguments to discourage it.

On the other hand, spamming or otherwise misusing email to deliver your unsolicited "message" at everyone else's expense is a classic instance of the Tragedy of the Commons. There is no incentive for any individual actor to do the right thing. Kant's categorical imperative is one of several reasonable ways to think about it.

If everybody visited one of this business website, the server wouldnt support. Yet, it is ethical that anyone visit. And if only people that really have an interest on visiting it do it. No problems.

I think the same for the email. If,everybody who can make a legitimate business proposal email them, no problem. And I apply the same ethics when I had my own company with a public email on my website (now im an employee and emails dont get to me)