Even good old fashioned IPv4 has fundamental issues and limitations (besides the obvious addressing shortage) which the global community is working through same like IPv6. The same reasons why people aren't using the latest Internet Protocol are also why they are still not keeping up to date on other technology changes as well. The scope of your question is way too broad to bother with addressing in detail here considering this is the fundamental protocol that the Internet uses (hence the name) so affects things from one end to the other and each would have its own list of reasons. So considering IPv6 has been around since 1998 & its only recently that we've run out of spare IPv4 addresses (even with using NATs to extend IPv4's life) and so finally have gotten some considerable traction then I wouldn't expect IPv4 to not be needed for a long time although we could see some IPv6-only networks with IPv4 gateways become somewhat common soon.
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Even good old fashioned IPv4 has fundamental issues and limitations (besides the obvious addressing shortage) which the global community is working through same like IPv6. The same reasons why people aren't using the latest Internet Protocol are also why they are still not keeping up to date on other technology changes as well. The scope of your question is way too broad to bother with addressing in detail here considering this is the fundamental protocol that the Internet uses (hence the name) so affects things from one end to the other and each would have its own list of reasons. So considering IPv6 has been around since 1998 & its only recently that we've run out of spare IPv4 addresses (even with using NATs to extend IPv4's life) and so finally have gotten some considerable traction then I wouldn't expect IPv4 to not be needed for a long time although we could see some IPv6-only networks with IPv4 gateways become somewhat common soon.