Choosing where to store observability data is not a simple infrastructure decision. It's a question about the essence of data. "Find the payment failure log user-789 saw on June 2 at 14:23" and "graph the 99th percentile API error rate over the last 5 minutes" are completely different questions, requiring completely different data structures. The first is a full-text search problem (finding specific documents in arbitrary text), the second is time-series aggregation (aggregating numbers over time axis). The former is solved by inverted index, the latter by time-series database (TSDB) — fundamentally different engines. That's why OpenSearch and Prometheus (AMP) exist separately.