true. none of smart contract blockchains today are actually a practical tool till lightclients that run on smartphones are mainstream

I don't think they are going to ever be mainstream. Even a lightnode that only downloads headers is going to eat up battery fast. If a centralized service works for you, what's the point? What's necessary is to allow actually persecuted people to run nodes on their computers.

Modern light clients don't use that much battery and are optimised for mobile usage. They even have very short start times making use of snapshotting and back filling headers i.e. you ask all nodes on the network that latest snapshot header close to the head and start from there working back towards genesis. This can be even faster than using an RPC and is far more decentralised.

Some ecosystems have made significant progress on in browser and mobile light clients. For example: https://github.com/paritytech/substrate-connect/

The problem is that Apple and Google have a kill switch.

The light clients that the parent envisions could potentially run entirely in javascript within a browser. No app store needed unless the browser engines start blacklisting bits of js.

Doesn't that leave them WIDE open to malicious code?