It's great that you got yourself such a deal. How is this even relevant in this discussion?
Low-mid range 500GB hover around $160 on Amazon, with high-end models easily reaching $250-300.
You can get a 1TB for $280, but that will be a slow 2.5" model (e.g. MX300). To get a 1TB NVMe you'll have to spend the $400 that I've mentioned or a bit more (e.g. Samsung 960EVO is $480).
And we should compare the Optane cache to NVMe drives, not the cheap SATA models. Why?
Because in typical usage scenario - when you boot the OS, check e-mail, browse web or start one of your 2-3 favourite games/applications - the "live" performance will remind that of Optane, not the slower HDD behind it. Everything needed often and quickly will be in the cache.
Yes, HDD will work in the background loading other OS files or game graphics, but this should not be noticeable - the ~100MB/s that modern HDDs offer is really enough.
Of course a pure-SSD setup has many advantages: no noise/vibrations and universally good performance - even when loading data that you seldom use. But you'd expect that given the huge price premium.
The popular SSD+HDD setup (SSD for OS, Documents and software), however, is not optimal, because you're wasting a lot of expensive SSD space.
In such scenario the SSD has to keep many irrelevant files (half of the Windows directory, most games' data, things in ./Documents that you haven't used for years etc).
And unless you have a large SSD (~1TB), you're limited by the size as well. You either fill it and are forced to move other files to HDD (less important games etc) or you end up using just a small part. As a result quite a lot of people have half of their SSD empty or filled with rubbish (mp3 or something).
Also, the small size could be a problem e.g. for avid Steam users. If you have a lot of Steam games, chances are that your Steam directory will not fit in your SSD. I'm not sure if you can divide it between multiple partitions (maybe via Windows links).
On the other hand, the idea of cache (Optane or not) is so much easier. You still live in a comfortable reality with big HDD-like partitions - you don't have to worry about space, move things around etc. But most of the time this setup is every bit as fast as NVMe SSD-only for a fraction of price.
In cache setup your fast and expensive SSD is always used optimally - storing just the things you need. Half of Windows, half of each game, the 1% files from ./Documents that you've opened this month and so on.