I think this misses the fact that the sharding spec contains multiple heuristics that allows users to get pretty good guarantees about data availability and correctness even in the case of a malicious 51% attacker. See:
As well as the already well-known concept of “fraud proofs” (or “fault proofs”), which allows clients to zero in on and self-verify incorrect blocks under the assumption (guaranteed by DAPs, described in the above linked article) that data is available. In the longer run, SNARKs/STARKs can be added to even further improve the security of these schemes.
Author: Vitalik Buterin
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