I believe the "small amount" is relative. What is consider small to you, may be consider large amounts to certain people. A farmer who considers $300 as large amount should be able to easily run full nodes and secure the amount rather than Doing that with online custodial wallet. It's a big risk to such farmer considering the amount involve.
The online custodial wallet should be for what Bitcoin users consider small and can afford to risk.
Nothing stops Network members from making it possible for every single users to easily run full node and secure their very valuable assets.
I believe Bitcoin should be Multi-chains like this: Interesting way to get millions of people run full nodes.
https://bitcointalk.org/index.php?topic=5224657.msg53809244#msg53809244Satoshi spoke about the idea of Sidechain(probably the first to mention the word), but wasn't detailed.