I wonder if game developers are actually disincentivized from qualitatively improving load times past a certain acceptable threshold, lest their AAA game be perceived as lightweight.

I doubt it. Gamers have been complaining about long load times since almost the dawn of time, and I think one with as little as technically possible could make it a selling point.

The poor market penetration of PCIe SSDs and their high price per GB is a strong disincentive for game developers to store data in lightly-compressed formats that require minimal CPU processing to load and use. We might see some improvement in using multiple threads to load game data now that CPU core counts are on the rise at the high end.

Reminds me of the first time I played Kings Quest from a RAM drive. I first played the game from a floppy disk, and it ways really nice to have everything load instantly!