Given the open source nature of the product, I'm not sure what vendor lock-in you mean. Migrating off cloud to your own infra again is doable. https://daylight.ai/blog/how-we-migrated-off-temporal-cloud-...
As our CEO and co-founder Samar Abbas stated in the keynote at this year's Replay conference (https://youtu.be/BxEB7Y2U9oU?t=532), Temporal Cloud is currently handling 35 billion workflows per day. If you assume that each Workflow averages 10 actions, that translates to more than 10 trillion actions per month.
Later in that keynote, a VP from OpenAI talks about the scale at which they are using Temporal Cloud. That'll give you a sense of the volume for a single customer (obviously a very large one).