Answer

I assume the Torah would be significantly different, since there would have been a response to changing reality. That of course depends on whether the Sanhedrin would also have had authority or only ordination, because part of the halakhic rigidity stems from the absence of authority, and therefore also of responsibility, on the part of halakhic decisors.
Defining it as a protective mechanism somewhat understates the matter. It is like saying that the requirement that judges be experts is meant to prevent non-expert judges from getting in. That is of course true, but it does not really capture the situation. You need experts because that is what is required in order to judge. Expertise is the mechanism that ensures expertise, and as is the way of Jewish law, the means become ends and can even become obstructive when there is no logic in it.
And finally, of course the way into the vineyard is not really blocked. Legal fictions like “they act as their agents” and the like provide us with workarounds. And if we did not have wicked secular people pulling our chestnuts out of the fire for us, they would adjudicate all the laws that need to be adjudicated under the rule of “they act as their agents,” and that’s that.