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While they're still shipping machines with < 1TB SSDs (read: for a long time), yes - why not take the relatively free space and I/O savings?

Because you can upsell a larger iCloud subscription to your customers using the new built in automatic cloud archival function. Then use compression server-side. $$$

On the other hand, people (sample size: my family and friends) tend to fill up their drives with already-compressed video and photos, which lz4 and similar cannot really compress further.

Yes, it can make I/O faster

lz4 really helps with anything that is compressible - if you store text or compressible data it can improve throughput up to 2-3x if not even more if it's not compressible it stops compressing early. On a modern CPU core compression/decompression throughput is in the range of multiple gigabytes/seconds.