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.
