Because I like Synology best - I find their software to be easiest to use and the most polished. And they have very decent support.
Also, since I'm already using their stuff, it's easier to stay in the same ecosystem than to make a huge revolution on all clients and in all workflows.
Why DS420+ over DS418? Mostly because of virtualization and more powerful SoC (it can index way more files).
It certainly is enough for my needs. I simply wouldn't utilize an even faster SoC or more RAM.
For someone wanting to run more VMs on a NAS or who has many clients, DS920+ is clearly a better option.
I'm OK with 1GbE as well. All my PCs connect over WiFi, which realistically makes 1GbE fast enough.
If I used LAN more, I'd probably think about link aggregation or would have waited for 2.5GbE.
Actually this was the cheapest option (by few $ but still).
4TB Iron Wolf costed me around $135 each (23% VAT included). They cost $106 on Amazon.
Putting aside enterprise drives (much more expensive), the only other 4TB option on Synology's compatibility list was Toshiba N300 ($140).
Most WD Reds from this generation were dropped because of SMR.
Why not 8TB? Two reasons:
1) it would be more expensive
My target was 10-12TB, which should last me until 2023 or so. For that I needed either 4x4TB or 2x8TB+4TB. The latter was 25% more expensive.
When the time comes, I'll replace two HDDs with 8TB. Who knows - maybe SSDs will be cheaper at that point.
2) I have plans for those 4TB once removed from NAS - they'll be used for backup.
So this is not just a single purchase for me. It has to fit a long-time strategy.
I have absolutely zero use for 10GbE right now and this won't change for another 5 years, if ever.