It seems to me that decoding FLoC IDs will be a big business, given their value. So people will do their best to reverse the system and keep an up-to-date table

It seems to me that decoding FLoC IDs will be a big business, given their value. So people will do their best to reverse the system and keep an up-to-date table for at least some of the more valuable/interesting cohort IDs, if not all of them. In the worst case you can do this just by paying volunteers with known interests and recording their FLoC IDs. Presumably that will be done enough that at least some of the data is available for sale. My further understanding is that FLoC IDs will be available to any website. So even if you’re too small to run a global tracking network, you can grab the FLoC ID from any visitor and, assuming that decoding services exist, turn that into useful data instantly.