Unfortunately miners don't get to make any real decisions about e.g. which transactions to include when running on a pool - so for arguments of decentralization

Unfortunately miners don't get to make any real decisions about e.g. which transactions to include when running on a pool - so for arguments of decentralization the pool is one miner. Of course the miners can move to a different pool if bad things start happening, but it's only reactionary. There's some neat tech that gets around this that is compatible with Bitcoin, for example p2pool. It's not super popular yet but it solves some of these problems.