You can use Trigger Mode, as per: Capture Modes | Saleae Support
(e.g., rising or falling edge, or a pulse will stop the capture)
As far as why gRPC, I’ll defer to Saleae for an official response. However, my guess is that it provides an ‘easy’ way to have a cross-platform & multi-language friendly API that doesn’t require low-level socket programming to access. The APIs are auto-generated for each language from a single/common protobuf definition. I think ‘easy’ is based on already being tooled up to support it at the higher level of API abstraction and on a full powered development PC, not at the bare bones IoT / lower-level networking socket layer creating packets directly.
Note: The gRPC site does list a lot of options: Supported languages | gRPC
… Saleae seems to only officially use python, but has talked about .NET and C# in the past. Another benefit of gRPC is an ambitious developer could create their own support on their own preferred OS/language without waiting for Saleae to do it for them. A higher barrier for entry, but more powerful once you get there (if you need those extra powers).
Perhaps when they add back UI automation API, it can be done more like you requested (e.g., simple socket-level control, maybe VISA and/or SCPI based vs. gRPC). Even better for some might be to restore the previous socket API and extend with new commands, if it fits in their future product roadmap.