I think the original question came up after the recent Rails keynote where they mention that, with NVMe speeds, disk is cheaper and almost as fast as memory, so Redis is not as vital.
https://youtu.be/iqXjGiQ_D-A?t=2836So Solid Cache and Solid Queue just use the database (MySQL), which uses NVMe.
So now, in addition to: "You don't need a queue, just use Postgres/MySQL", we have "You don't need a cache, just use Postgres/MySQL"