The reason nothing has replaced email yet, even tough it's an old and clonky protocol is because it's an open standard and people don't make standards any more. Today you make services (most often based on HTTP because it's a protocol any 20yr old junior programmer can handle) that are locked in, harvesting has much data about it's users as possible desperately hoping that it could monetize on this data one day.
And don't forget when the company can't monetize data well enough (because the only people making money on this garbage are the ones with 20% of the global populace) the company goes away, if you're lucky gives you a way to archive your whatever or fetch it via an API, after which you either are locked into the Shiny And New service that everybody likes because it's shiny and new, or have a massive XML/text/zip file full of garbage you know you're never going to take the time to sort, and rinse and repeat.
Related to that point: the fact that email still provides a relatively low overhead method of reading and sending communications, and the end-user is somewhat in control of the user experience because they can choose what email client(s) they want to use.
Nearly every other 'email killer' gets both of these things badly wrong.
