Not even close. HDDs are for storage, SSDs are for the system. Is it THAT hard to understand? I hate to read this stuff over and over again. HDDs are CHEAP. You couldn't possibly afford a 50TB NAS storage filled with SSDs.
Umm, it is hard to understand because what you say makes zero sense and is inaccurate.
HDD and SSD are both for storage and they're both for the systems.
What you are trying to say is:
1. Hard drive is still great for low-consumption/medium-term archival type of storage that does not require a lot of writes. However, hard drive do degrade over time via bitrot, so it is not for long term storage. Best to keep backing up your data at least twice. It is also recommended to replace your hard drives once it exceed 5 years of usage. It is vital that you verify your data backups twice a year.
2. SLC/MLC SSD is excellent for the main/primary drive for daily consumption, while HDD or even hybrid SSD/HDD is still much cheaper for low use storage.
TLC or higher *LC SSD will replace HDD over time as it can store much more data in the same space as SLC/MLC SSDs at a much lower price with a lower performance grade.
They're working on stacked or 3D NAND chips that will enable them to archive even greater storage at lower prices as well. Intel announced one a few weeks ago that can enable the path to a 10TB SSD within a year or two but no news on what the expected price are and it is not yet known if it will be TLC or MLC.
Prices will decline over time for SSDs, it already is reaching $.40 per GB for some SSD. You can grab a 1TB SSD for 360$, that isn't true 2 years ago as it would've cost $600 and even more 5 years ago that exceeded $1 per GB. Granted, the archival 8TB drives are 300$ now. HDD itself peaked at the bit density, it is getting almost too difficult to keep ramping up the density. SSD is definitely not peaking anytime soon as we still have 5-10 years worth of technologies coming that can increase the density and decrease the costs. This is why HDD will die out, we will get 10TB SSD faster than we will get 10TB HDD and prices will drop over time to reach $.3 per GB or lower as soon as the stack/3D NAND is ready.
The screenshot above is from Anandtech.