So here's another example of how low Izuku's self worth is. While Izuku has had setbacks (mostly injuries to himself), this fight against Shouto is Izuku's first real loss since All Might became his mentor. And Izuku immediately thinks All Might wants to replace him.

All Might using Shouto as a reference point for how One for All would work for someone with another quirk only solidifies to Izuku that he might want Shouto as a replacement. He has the backstory and the power of a main character after all. (It's obviously unintentional on All Might's part. That man couldn't read the room to save his life.)