I bought my last SSD in December 2012, which is a long time for me to go without an upgrade. There are many valid reasons for holding off the upgrade.
Firstly, the SSD prices haven't come down as much as I expected in the last 3 years. There was a huge drop in price before 2012, annually. Conversely, the Japanese Yen has gotten weaker in the last couple of years. The overall effect of this has been such that a 250GB SSD nowadays is more expensive (JPY 15,220 for Samsung 850 Evo as of today) than what I paid for (JPY13,800) my Samsung 840 250GB in 2012. So my reluctance is natural.
My Samsung 840 order from Sofmap
Secondly, even though the benchmark applications may be able to differentiate SSDs by their performance, it is almost impossible to do so while actually using it. All SSDs feel fast. Sure you might be able to launch applications milliseconds earlier than others with a faster and more expensive SSD, but that's hardly something that can force someone to upgrade.
Thirdly, I hardly have my SSD filled with data. I only install software on it, including games and I keep all my data on the regular hard drives. While games have increased in size significantly in the last couple of years, some even exceeding 50GB such as Titanfall and Battlefield Hardline supposedly requiring 60GB, I don't play a lot of games. In fact, there is only one game installed in my SSD and that's Crysis 3 (check out my YouTube gameplay video playlist here). I don't feel like playing anything else as I thoroughly enjoy playing Crysis over the internet. So, I'm hardly having issues with respect to capacity as you can see from the screenshot below.
Only 40GB filled out of the 232GB of my Samsung 840 as of this moment
In my opinion, the only reason why I would need to upgrade the SSD is death of the drive. If that happens, I've got not choice but to buy a new drive. However I doubt I would have to face that in a long time, because my drive is doing really well in its health department, despite containing TLC flash. Of course, if the drive dies, I would look at a drive that's much faster than my current drive, even if it only in benchmarks. Whether to buy a single large drive or a couple of smaller drives to RAID them and double the speed, is a discussion for a later date. But if you are going for 512GB or more, it is better to get a couple of smaller SSDs. If you are going for less than 512GB, I would settle for a single drive.
That said, I'm not saying that I don't feel like getting a couple of Samsung 850 Evos now and setting them up as RAID-0. I wish I could do that, but unfortunately the cost is too much. Impulse buying is not out of the equation still.