If one had the experience of some 30 years from the beginning of what we once called "micro-computers," it might put all this in a balanced context.
In the early 1990s, I know that I and others were trying to hold onto PCs with 5 or 6 years of prior usage. Sometime in the mid-90s, somebody told me "you could afford to buy a new PC every year." In those days, a system from Gateway delivered to your doorstep might have cost you between $2,000 and $3,000. I think I started building PCs myself around that time, and it seemed much less expensive.
But since the time we migrated from IDE drives to SATA -- a time when you might buy a drive of 160GB -- I've only had one HDD go south. It was still within a 5-year warranty, and the manufacturer replaced it.
I have in my parts-lockers one or two of those IDE drives left, and a couple 320GB "WD Blue" SATA drives. On top of that, I have some 7 or 8 hot-swap caddies that fit bays in all our machines here, with either 500GB or 1TB size. I use them for backup, but I don't need them for backups. I probably need at most about three for backup, because all our systems back up to our server at 1AM every morning.
Yet, what one respondent said is true: HDDs are more prone to failure simply because they are "electro-MECHANICAL" devices.
Now consider the types of SSDs available. For instance, I just recently purchased an ADATA SP550 480GB SSD for $110. Its MTBF is spec'd at 1.5 million hours, which translates to 171 years (I need to pinch myself and recheck my Windows calculator, but that's what it tells me!)
MTBF isn't some hard and fast limit: not all SP550s will last 171 years. Some will last longer, some will expire sooner. It's an average and an estimate. Around that average or "mean" (for "M") there is a distribution of expected failures.
The other measure of expected lifespan is the number of expected writes in terabytes or petabytes before one experiences performance degradation or failure. And these measures or estimates are reported in various endurance tests, such as one published by TechReport and cited here in other threads. I think several drives could be expected to allow as many as 500TB of writes, and one drive -- a Samsung -- scored at approximately 1.2 Petabytes.
But the drive with specs similar to the SP550 cost me (as a Samsung 840 Pro) about $400 in 2014. Of course, the Sammy was an MLC drive, and the SP550 is TLC; people have more reservations about TLC.
The warranty period for this type of device doesn't correspond to either MTBF or cumulative writes. 3 years? 5 years? I could easily see discarding the SP550 at some point before 5 years. It would've cost me $20/annum.
The entire culture that has emerged since the IBM-PC-AT days has gravitated toward throw-away devices. Your cell-phone provider may give you a new I-phone for $50 upon accepting a new contract; it's your own duty to recycle the old one. Last year's Surface may give way to next year's tablet.
I don't much like the way this is shaping up. I never imagined in 1990 that computing devices would become such a fixed part of my annual budget, but they have. Yet, the devices are much cheaper.
My SP550 may have as many as 60GB of writes per day, and I would strive to reduce that figure, if only for strategically choosing to use SSD's as boot-system disks with a profile of more reads and less writes. But in the overall scheme of things, it doesn't bode much to worry about.