I doubt that the VoLTE requirement was put in for Airwave's benefit. The emergency services customers rely on various features of TETRA that are not available on LTE - that is a serious risk, not just a procedural change.
These are not mobile phones, they are not used for pure point to point conversations - and the timing requirements for TETRA are quite tight - you don't press a button and wait ten seconds before you can start a conversation.
That sort of thing can be really important if you are under attack, or in a burning building, so it isn't "most of the time", it's "all the time".
Similarly a call for help will be heard by many local officers as well as the control room - allowing someone who is just around the corner to respond without delay.