Re: "Limited writes haven't been a problem for good solid-state storage for 10 years or so..."
That's not true.
Newer Flash based SSDs are based on MLC flash and have have lower write endurance than older SLC based models (~10x lower). Firmware on these SSDs distributes the writes across the entire capacity of the SSD, this is called "wear-leveling". Bottom line is that all Flash SSDs wear out with write activity, and the more the density of the underlying Flash parts increases (from SLC to 2-bits-per-cel MLC to 3-bits-per-cel, etc), the faster the Flash wears out.
Disk drives wear out with time, Flash wears out with usage.
Flash is NOT a good place to put a file-system log, not only because Flash wears out, but for economic reasons. The log/journal writes are 100% pure sequential I/O (no randomness). For this workload, spinning disk costs only about 1/10th as much as SSD in terms of cost-per-MByte-per-second.