MS won't do this because Live is their big seller and their big money maker. Their 1st party titles have very weak brand value, with the only one even close to a household name being Halo. People buy XBox because they want to play 3rd party titles like COD or Madden or GTA with their friends, and their friends are on Live because launched first and launched cheapest last gen. If there was cross-platform online match-making, there would be little reason to buy the XBOne. Hell, even the PS4 would become a tougher sell when you can pick up a Wii U cheaper and play online for free. And with Sony moving to a paid online model, they too have reason to keep you on their network and try to convince you that it's worth $50 a year. Tougher sell if you are playing the same game against people who are paying nothing.
I would love it if it became widespread, however. What a world it would be if I could pay no subscription fees but play against my friends on XBox and PS.