Use metrics & SLOs to help diagnose the health of your systems. Derive those directly from logs/traces, keep a sample of the raw data, and now you can point any alert to the sampled data to help go about understanding a client-facing issue.
But, for auditing of a particular transaction, you don't need full indexing of the events? You need a transactional journal for every account/user, likely with a well-defined schema to describe successful changes and failed attempts. Perhaps these come from the same stream of data as the observability tooling, but I can only imagine it must be a much smaller subset of the 100PB that you can avoid doing full inverse indexes on this, because your search pattern is simply answering "what happened to this transaction?"