The death-knell in my opinion for true decentralization of the bitcoin network (ignoring mining for a moment) happened when the blockchain became too large for it to be practical for normal users to have the whole blockchain on their machine and operate a real node. I am sure there is some creative solution to this, perhaps an additional mining process by which old records are combined to save space (i.e. transactions older than a week get squashed so that only the resulting balance is stored instead of the individual transactions, perhaps). It might also help to not even store information about a wallet address on the blockchain unless it has a nonzero balance. I have no idea how this is actually implemented so this is all speculation.
You can easily store the blockchain (which is mostly historical data, ie. “empty accounts”) on an old (rotational) HDD because you access it in a sequential manner, which is fine for old HDDs. It’s only the set of “non-empty accounts” (unspent transaction outputs in technical terms) that you need random access to. But this data can be stored in RAM since it’s only about 3.5GiB.
Anything over 40 GB is impractical for wide-scale distribution that would create a healthy network, and really anything over 2 GB is problematic and will have a much narrower reach.
This is evidenced by the fact that almost no one these days actually runs a local wallet. The number of logical nodes has decreased while the number of wallets has dramatically increased.
