block size does not harm decentralization, how block size is increased can do that. for example if you increase it suddenly to a much bigger number now it will end up centralizing bitcoin but if it is increased slowly with the advancement of hardware and internet speed then it can't do much harm.

the most important thing to remember is that size of the blockchain is only one of the problems. the bigger problems is the hardware limitations. if we have 10 MB blocks for example you will have to verify 10 MB worth of transactions every 10 minutes on average and also be able to receive and send that much every 10 minutes. additionally miners will start facing problems, specially those with slower internet speeds who have to compete with those with high speed connections and that leads to miner centralization.
so the numbers you are reciting above (13.33kb/s, etc) are only when you assume people download 1 block per 10 minute but that doesn't make any sense. not for miners not for others. and it is not just downloading blocks, a node does a lot of other things that takes traffic including relaying transactions in its mempool