If everyone would do it like that, the file would never be pulled from the CDN at all (since a cache miss pulls from the local src), therefore the try-shared attribute would never be useful.
Note the proposed scheme doesn't prevent anyone from using a CDN as the src while also trying other CDNs to increase the chances of a cache hit, which has its own benefits.
For example, <script src="https://code.jquery.com/jquery-3.4.1.min.js" try-shared="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.mi... try-shared="another-src">