Satoshi didn't intend for ordinary users to run full nodes. Miners would specialize in their field and regular users would not be forced to run anything.
The decentralization of the Bitcoin network was supposed to happen spontaneously and be encouraged by the use of capital incentives already included in the first version. This also necessitates keeping transactions on the main chain, which is likely why the specific term "decentralization" was never used in the white paper.
Aside from this, Mores law and an interest in developing better technology, if not directly for Bitcoin itself then for other industries, was expected to help with any limits to the scalability of the chain. With time, the security of the network will increase both in terms of hashing power and aligned interest.
If by some chance, for any reason, this still is not enough in the future it is of course always possible to fork again. But if venturing outside of what is properly called "Bitcoin: A peer to peer electronic cash system" and which fundamentals were detailed by Satoshi himself, it can no longer be called Bitcoin.