The only possible lock-in I see is if you have annotated code with Datadog specific tracing instructions and I'm pretty sure that most of these features aren't relevant for Open Telemetry.

Switching APM providers isn't hard at all. Maybe the Open Telemetry ecosystem lacks a good agent but my understanding is that their APM agent is pretty much already a copy of the Datadog agent.

I think New Relic would be a more interesting agent to use because their approach to APM isn't primarily to do sampled traces.