The reason there can be no evidence for design is because such evidence is subjective. Two people can look at a tree. One thinks it is evidence of design. Another says it is just a result of biological processes, cell growth and division.
Scientific evidence is information that can be gathered, held, and examined. We see cells divide. We can see the fossil record and can analyze similarities and differences between fossils. We can analyze how organisms are related genetically. There may be argument about the interpretation of data and what it means, but the data is there. How does one gather data and quantify design? If you can come up with a way, that would be the greatest discovery in all of history. But no one has done so. That is a matter left to faith.
So even if we say that something in nature is designed, that all of the data gathered about it is dependent upon that statement of design, it still doesn't change how it is going to be studied. The chemistry, biology, and physics will continue to be studied as it had been before.
That is why the scientific establishment opposes ID. It will continue to oppose it and will defeat it eventually, rightfully so. What is going on is that people who oppose the left are being taken advantage of by those who, for whatever reason, cannot accept evolution. You're argument is with the left.
It's not true that the scientific establishment is disallowing people to discuss this issue. It is being discussed right now. Yet the rejection of ID will not result in anything beyond some people's feelings being temporarily hurt. No one will be killed or exiled. The rejection of an idea is not oppression. But someone has to win.
Science has already produced detailed explanations of a large number of self-organizing processes.