I've been out for a couple of years, but...

> I'm honestly not sure what sapling is

My rough understanding: arcanist + git evolved into arcanist + hg, hg became a frontend to cHg and then the whole edenfs/sapling stuff started to replace that to optimize sparse checkout workflows.

> - Buck should've been killed and replaced by Bazel. I can't speak to Buck2 but this seems like a pointless investment;

Bazel's open-source version remained significantly less capable than Buck for a long time; Buck migrated towards Skylark just like Blaze, and cleaned things up greatly.

> - Thrift should be killed and replaced with gRPC/Protobuf. Same deal;

In general, agree with the former, but that's because protobufs and especially flatbuffers were already used in many places for many many years.

> FB's code search is just grep

There was also a less popular semantic search that people didn't use nearly as much.

> There's no dependency declaration at all

Strictly speaking this is not true, it was just not usually needed.

> There is no way to say "if this file is modified, run this test".

You needed to update the target determinator, but I do think this was possible.