Simple answer:
Satoshi never knew crypto will be this important.
Nope. Read his e-mail of November 2008:
http://satoshi.nakamotoinstitute.org/emails/cryptography/2/He understood perfectly how it would go.
The only thing he missed, was the collaboration of mining into pools. He didn't realize that people could pool together, and sell part of their hashing effort into a fraction of a block reward, so in his opinion, there would have been more individual miner nodes.
That said, mining itself is automatically limited to a finite number of sensible participants. It wouldn't make sense to be 1 million independent mining nodes (the only nodes Satoshi considered, were mining nodes: he never considered non-mining full nodes as meaningful - and he was right). The reason that it doesn't make much sense to have 1 million mining nodes, is the fact that there are only 52 600 blocks mined per year.
Even if there were 1 million mining nodes with all having exactly the same hash rate, that would mean that *on average* a mining node would win ONE SINGLE BLOCK every 20 years. But given statistical fluctuations, you might as well mine for 50 years and never have a block. Or you might mine for 3 years and be lucky and find a block.
Given that all capital has a power law distribution, and never a uniform distribution, even if you would only have 5000 mining nodes, where, with a uniform distribution, each node would, on average, mine 10 blocks per year, the power law distribution would give most blocks to the 1000 upper class miners, and almost never a block to each of the 4000 others.
So it would, in any case, in Satoshi's mind, not have been meaningful, statistically, to have many thousands of nodes.
The strategy of pooling together has of course strongly amplified this effect, and he wasn't aware of that. In a certain way, mining pools add a small sniff of decentralization to the, in any case, obvious centralization that the economies of scale would induce in the competition for mining. If mining pools weren't possible, we would quite possibly simply have 5 or 10 mining nodes, period. Who can, effectively, still compete in *solo mining* as Satoshi saw it ? The industrial proportions of mining make solo mining only available to a big company. You'd have Intel vs Motorola vs Nvidia vs AMD or something. Now, with mining pools, even with a modest mining investment, you can still contribute. However, the "voting power" of a mining-subcontractor is very small: he can just go to another mining pool. But it is better than nothing.