Yeah, "Hard drives are the root of all evil." Only, not necessarily. There are many potential causes for a slow Mac (my favorite culprit tends to be bad startup/login items, and near-full disk is another). Sure, SMART shows 4 reallocated sectors. 0 errors, 0 CRC errors. Every entry in the "Failed" column is, "Never." That's hardly the end of the world. There are roughly 2
million sectors per GB of storage (at 512 bytes per sector), so since we're talking about a 1TB HDD... four out of 2 billion sectors? Care to calculate the percentage?
Sure, if I was running a data center, I'd probably be all over SMART status, and be pro-actively replacing hot-swappable drives right and left. But a hard drive replacement in a 3-4 year old IMac? Not the first thing I'd recommend due to its cost and complexity, especially if a restore from Time Machine backup afterwards might (or might not) resurrect some of the original problems.
Here's the thing about status-monitoring software. Judgements like "failing" are just that - judgements. What criteria go into those judgements? What is the target market of the product? What is "Aging" and "Failing?"
https://www.volitans-software.com/support/smart-utility-faq/ - having read that FAQ...
In the test results the OP shared, all those "aging" and "failing" labels in the Type column are accompanied by a "Never" in the "Failed" column. Yet despite all those "Nevers," the overall result is "Failing?" Until there is a value other than "Never" in "Failed," then, by the developer's own definition, that particular item is neither aging nor failed.
Yeah, the developer believes that having just a few bad sectors means the drive is failing. Yet he says, "Some people had drives that with a few bad sectors, that lasted for years but in my experience, when a couple of bad sectors are found, more will follow." Yes, more will likely follow. That's the way the world works. I've been having a few "senior moments" lately. It must be time for assisted suicide.
Again, he says, "In the end, it is up to you whether to replace the drive or to take a chance with your data on that drive, hoping that it will last for years." Seems pretty dire, except this is exactly why we make backups. If your customer base is made up of people who are worried about failing hard drives, you're probably better off playing to those fears than saying, "Nothing to worry about, just make sure you have a good backup!"
I've yet to replace the HDD in my Early 2008 iMac. Been running betas on that thing for years, had plenty of slowness issues - did an erase/reinstall OS X/restore from Time Machine a few months back - big help. At other times, Etrecheck found a number of software-based issues that also made a huge difference. Altogether, the thing is performing better in 2016 than it did in 2013 when it was demoted to "Mac #2."
I'm not saying my issues and solutions will necessarily be the next person's issues and solutions. However, these are complex systems - simplistic approaches like, "When in doubt, blame the HDD" just don't cut it.