1. Just like many, many things that used to have supernatural explanations (lightning, disease, the building of the pyramids, etc.) we will discover a natural explanation in time. This has been the trend throughout history.
2. We can never know this, but let’s assume that we will never figure out an explanation for first life. This still does not mean that the explanation is supernatural. There may be wholly natural aspects of the universe that are fundamentally outside our ability to understand. Stating that the supernatural is the most likely explanation is to assume that we are even capable of comprehending the entirety of the natural universe - which seems unlikely to me, since we are part of the natural universe ourselves.