As far as I know, the AI designer is busy. I assume there are regular "rounds" where the AI checks for new tech, so new designs are not instant on new research available, but it will be done rather soon.
If there are no new components, there won't be new designs.
As for the private sector retrofitting, who knows? You can see the distribution of designs in use from design screen. Just make sure to display all designs, (also obsolete ones), and I would suggest to sort by subrole.
From my empire I can see that of course, all mine designs are in use, as they can't retrofit. For private ships, the oldest appears to be out of use, and that for the 4 versions of each, it appears that the large freighters are most eager to upgrade (or most eager to retire the old).
It would not surprise me if ships that are busier (more profitable) retrofit first. I would also not be surprised if large freighters are the most profitable. I have spotted freighters on a retire order when I have poked around the traffic around space ports.