4 HDDs can do 200-400MB/s _sequential_ IO, 1 modern SSD can do 150-200MB/s _random_ IO and 400MB/s sequential IO while 4 HDDs would have a hard time doing IIRC more than 8MB/s random IO

i don't argue that. It is just random IO isn't that important for our use case, especially compare to capacity and sequential IO which are important for us and which HDD is ok for.

Urm what? A modern NVMe drive will sustain ~2 GB/sec write.

(See e.g. https://cdn.mos.cms.futurecdn.net/Ko5Grx7WzFZAXk6do4SSf8-128..., from Tom's Hardware)

Few SSDs can sustain such a speed for a long time. After they exhaust their temporary SLC cache they drop to much lower speeds. SSDs that have accumulated a large amount of writes might also make large pauses at random times during writing, for garbage collection. Good SSDs remain faster than HDDs even in these circumstances, but they are nevertheless much slower than when benchmarked for short times while they are new.

Note how, in the graph, even the worst-performing SSD stays above 500 MB/sec sequentially for an indefinite amount of time, while the parent post claimed SSDs couldn't even do 200–400.

You can get SSDs without such issues. At least for the kind of long sequential writes followed by deletes where HDD are are even vaguely comparable, good SSD’s can maintain very close to their theoretical maximum speeds indefinitely.

They only really slow down in usage patterns that trash traditional HDD’s, which have much worse disk fragmentation issues.

And that's why you buy 2-bit MLC, which can sustain these writes forever. Like e.g. the 970 Pro.