You could absolutely treat phoenix.new as your full dev IDE environment, but I think about it less an IDE, and more a remote runtime where agents get work done that you pop into as needed. Or another way to think about it, the agent doesn't care or need the vscode IDE or xterm. They are purely conveniences for us meaty humans.
For me, something like this is the future of programming. Agents fiddling away and we pop in to see what's going on or work on things they aren't well suited for.
Tidewave is focused on improving your local dev experience while we sit on the infra/remote agent/codex/devin/jules side of the fence. Tidewave also has a MCP server which Phoenix.new could integrate with that runs inside your app itself.