It's looking like 2029 will be the turning point. Right now, we are on the verge of having 16tb m.2s on the market, and by 2029 SSDs will be around $10-15/TB like HDDs are now.
In 2029, if semiconductor trends continue, it is likely that we will have 16TB SSDs for ~$200 and 32TB SSDs for ~$500; Cheaper than the $320 we're paying for 20TB HDDs right now.
The HDD industry doesn't seem like it will improve at the same rate. It is likely that the SSD market will have better $/TB than the HDD market in 2029, unless hard drives make some massive breakthrough before then. The survival of the HDD industry past the next 5 years is basically riding on Seagate's ability to successfully release HAMR technology.
While I fully agree with the SSD side, you seem to ignore that HDDs are also getting cheaper per TB (always have, and usually quite noticeably). Also the reliability of large to huge SSDs remains to be seen as well. Obviously a breakthrough in HDD technology would have an influence as well, as you mentioned.
I'm not saying SSDs aren't here to take over, they surely will eventually (preferably sooner), but I think it'll be a few more years until we got actual price parity per TB. Even when ignoring other aspects like reliability.
You can’t really reliably use consumer SSDs in a server/NAS situation though, unless you more prepared to replace them every 12-24 months and suffer poor read/write speeds under load
I’m not sure although it’s mostly used for media storage so there aren’t a lot of write operations. Having said that I do have solid state M2 drives in there for caching with no issues so far.