I2S is not a standard for transmitting data, it is a standard for serial transmission much like 24bit Least Significant Bit First, or the opposite 24bit MSB first.
The S/PDIF standard provides information about how and what. It specifies +/-0.5V, a 75ohm line, stupidly an RCA connector, and how the data is sorted.
I2S has no specs for data and connections since it has no exclusive purpose in audio; I2S is also used by microcontrollers to program filters, or managed HDCD decoders. It does however spec things like MSB first, and that the MSB should arrive 1 clock beat after the L/R clock changes, and finish one clock beat before it changes again.
What would be idea is to have a new standard (did I just say another one?) on top of I2S which has specifications for interconnects between audio connectors like DIN etc.
Yes I2S is a lot better then S/PDIF since it has several discrete data lines and clock lines. All the problems of S/PDIF could be solved if it is terminated correctly and if it did not multiplex the data and clock lines for transmission.