The best replacement for spdif is optical.

The best alternatives to spdif are rated as following:
★★★★★  optical
★★★★  hdmi
★★★★  rca
★★★★  toslink
★★★  usb
★★  coaxial
★  adat
★  pcm
★  aes
★  usb audio
★  3.5mm
★  aes ebu
★  usb dac
  hdmi audio
  i2s
  coax
  optical cable
  optical audio
  rca cable
  analog audio
  digital coax
  optical spdif
  digital optical
  aes/ebu

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Google Trends

Scale is based on the average worldwide traffic of spdif in all years. Click on graph for more.
spdif
optical
hdmi