this is the nature of all the PoW coins. The early contributors take the most risk and the most reward (if the coin succeeds).
No, Satoshi did not "premine" bitcoin. Premine means on day zero you have created currency for yourself already. The only block we know Satoshi did mine cannot be spent. There is also zero evidence that Satoshi mined additional blocks once the network got up and running. (It took days for the second block to be found.)
Early adopters invested almost nothing compared to the amount of money moving around now (at risk on a speculative investment). Their risk was negligible and certainly far less than folks getting into the game now, coin for coin.
You’re right that if one buys 0.05 btc today for $400, it’s the same risk as someone investing $400 years ago to mine hundreds or even thousands of coins, but real investors aren’t buying 0.05 btc.
The difficulty curve often further benefits early miners to be taking minimal risk when the design is reverse logarithmic.