~8 years is what SSDlife defaults to when it doesn't have enough data to calculate lifetime. My understanding is that it uses the rate at which the media wearout indicator decreases to extrapolate lifetime. Since the MWI is linear it's an easy calculation. For example if it takes 10 months of usage for the MWI to decrease by 1%, then assuming your usage stays the same it will take another 980 months (81.7 years) for it to decrease all the way to 1%. If your disk is still sitting at 100% MWI and hasn't dropped to 99% or whatever since SSDlife has been installed and running, though, it will have no idea the rate at which your MWI is decreasing and cannot calculate lifetime. But instead of just saying there's insufficient data or something like that, the program just defaults to a phony number.
And technically it's not really a rating of lifetime, it's just an extrapolation of how long it will take at your current usage before the MWI on your drive reaches 1. There are other ways an SSD can fail long before you'd have to worry about exhausting MWI, such as hardware component failure or a firmware bug that bricks the drive.
That and your SSD more than likely won't stop working even when MWI hits 1 and all the writes on the flash are supposedly exhausted. In reality NAND can usually handle a good deal more writes than it's rated for (TLC rated for 1000 P/E cycles may actually be able to handle 2000-3000 or more for example).
Only reason I really use SSDlife is because it has a lot of useful info (MWI, power on hours, power on times, etc.) all in one place and in an easy to read format. The estimated lifetime is completely useless in my opinion, though, I wouldn't pay much attention to it.