You're essentially describing a long range attack. That is the tradeoff that PoS makes over PoW's full objectivity, in that a node who comes online for the first time (or after being offline for a while) cannot fully-objectively distinguish between the real fork and an attacker's fork who simulated years of attestations.
It's where the term "weak subjectivity" comes into play, you just need a recent state root from a node you trust (be it a friend running a node or a block explorer or whatever) and that's what will tell your node which fork is the real one, and from there you can just keep following attestations from real validators and the long-range attackers can't hurt you
You're essentially describing a long range attack. That is the tradeoff that PoS makes over PoW's full objectivity, in that a node who comes online for the first time (or after being offline for a while) cannot fully-objectively distinguish between the real fork and an attacker's fork who simulated years of attestations.
It's where the term "weak subjectivity" comes into play, you just need a recent state root from a node you trust (be it a friend running a node or a block explorer or whatever) and that's what will tell your node which fork is the real one, and from there you can just keep following attestations from real validators and the long-range attackers can't hurt you