This is not a problem the average user is having. Anyone remember the "IM wars"? Most people happily installed all IM programs their friends were on, thus solving the problem. Only a small handful of nerds thought this was intolerable, and installed Adium or Trillian or Miranda or any of the other multi-network instant messengers, and suffered every time an actual network would upgrade or change their protocol.
This time around, people's smartphones are already the unifier. All messaging apps have notifications, so your phone will tell you what everyone is saying across networks, and you just tap to go straight into whatever app the message was in.