It is possible to build a completely decentralized application, but how exactly to do this and on which DLT platform: Ethereum, Quorum, Hyperledger Fabric or whatever depends on your specific business case.
Here's an ethresear.ch thread covering the same topic, for anyone who may stumble upon this question (as I did): https://ethresear.ch/t/clarification-on-how-calldata-persists-on-the-blockchain-and-how-optimistic-rollups-use-it/8136
Some answers from that thread:
(barrywhitehat) Call data is stored as part of the eth chain history. If you want to validate ...