That is the biggest problem as far as I am concerned.

Back in the day when people... you know... talked each other, it did not matter what phone company you were registered with or even what country you lived in. You could just pick up the phone and dial the number and hey presto, you can talk to your sister who is married in Switzerland. You did not need to care that you were with British Telecom and she was with Swiss Telekommen or whatever they call it there. The same still applies with emails. So why not instant messaging?

"Decent alternatives" my a55. If I could use my CoolApp to chat with my sister using KewlApp then sure, I would use a "decent alternative". But since they are all mutually incompatible, I can only use the only one that we both agree on. And that means Skype and nothing else.