Why can't Promise be completely replaced by async/await? In what situations is async/await not suitable?

Async/await is syntactic sugar over Promise that simplifies working with asynchronous code, making it more readable and similar to synchronous code. However, you cannot completely abandon Promise for several reasons:

1. Parallel execution: async/await by default waits sequentially for each operation to complete. If you need to run multiple asynchronous tasks in parallel and wait for all of them, it's more convenient to use Promise.all or other Promise methods.

// Parallel execution with Promise.all
const [data1, data2] = await Promise.all([fetch(url1), fetch(url2)]);

2. Composition and chaining: Promises make it easy to build chains and combine asynchronous operations without needing to write many async functions.

3. Error handling in chains: sometimes it's more convenient to use catch on a Promise, especially in complex chains.

4. APIs returning Promises: many libraries and APIs return Promises, and using them directly is often simpler.

5. Event-driven programming: async/await is not suitable for handling events where you need to respond to multiple events at different times.

Thus, async/await is excellent for simplifying sequential asynchronous code, but for parallelism, complex chains, and event-driven programming, Promises remain necessary.