
This is about 5x faster than a mechanical hard drive in terms of reading & writing. Access time is ridiculously low 100x
How much storage, compared to cost?
You can't entirely replace HDDs - it's not practical.
If you hardly do anything with a computer, actually, you can. If you actually like to store anything, and install anything - well, those who need storage space know what they need.
I have a 256GB SSD for the OS, some apps, some games.
I also have over 2TB of HDD storage space as well (some in a RAID 0 volume, some in a RAID 1 volume). More games, downloads, documents, photos, etc... go on there.
Also - you still have to be careful with SSDs. You shouldn't keep them full or even close to it, as that limits the ability of TRIM and helps lead to faster degradation of memory cells. Especially if it's your only drive, or if it's your boot drive but you haven't configured anything deeper - you'll have caches and temporary storage on there, and that eats up precious write cycles. Will you kill it in a year? Highly unlikely. Will you kill it in five years? Borderline to likely if you are a heavy user without precaution. Can you make SSDs last over 10 years? Sure... possibly.
They have great use, but it shouldn't be for critical storage. Waste of money to use them for storage, for one thing. Documents and files usually aren't large enough to create slow loading - and in those cases, critically important large files are probably part of a production environment, and such files should be on nothing less than a RAID 5 storage solution. Even better, RAID 5 or higher on 10k SAS drives.

It won't be SSD speed, but with the right SAS controller and HDDs, a good RAID configuration can get you close enough to make working with those files impossible to tell the difference. The main loading issues most of us experience are the applications themselves, which is a great use for SSD.