It's a mistake IMO for old hats like myself. After 20 years using ifconfig, it's almost impossible to get used to some new, linux-only tool (even though it may be better).
It's pretty rare that you actually need to use ifconfig/ip directly in Linux. For every distribution that I know of there's a specific way to setup network interfaces (e.g. /etc/network/interfaces, /etc/sysconfig/network-scripts/ifcfg-*, etc.)
The "ip" command has been the preferred way to configure network interfaces in Linux for a few years now. Most functionality isn't available through "ifconfig" at all.
I'm willing to wager that 10 years from now - configuring an interface will still be done through ifconfig.