I think you are right and data model should have been event based rather than span based. https://medium.com/opentracing/open-for-event-based-tracing-... digs into that topic and basically says that distributed tracing is about causality & partial ordering which bring us back to the 70's with Lamport's logical clocks and all following work.

However, this design "flaw" is well known and seems assumed. I'm not able to find relevant GitHub issues right now, but I remember this topic being discussed on OpenTracing or OpenTelemetry bug tracker and the outcome was something like "Spans might not be the best data model, but people are now used to it and we have to ship the spec within a reasonable time, so let's stick to it".

Edit: https://github.com/open-telemetry/opentelemetry-specificatio... might be relevant to your concerns.