In my opinion, interop should be instant with a turn of a dial. I keep hearing about the use of ACU's, etc. those things take time to deploy. Plus, what if the incident suddenly becomes mobile and out of local jurisdiction. Read up on the Boston marathon bombing, many agencies involved and all utilizing the BAPERN network. The system was introduced in the 70's and after the incident it was noted how well that system still works, as they had flawless communications with all agencies involved.
You bring up a couple good points.
BAPERN works because it was developed, like similar things, by people who understood how to work together. The system also uses a single band that everybody in the area has radios for. That by itself doesn't force everyone to use that single band for day to day intra-agency communications, but everyone knows when you need help that's the radio you grab.
I feel that is a very good model for other places to follow. I disagree with some people that feel every agency needs to be on the same band all the time, ala FirstNet on 700 MHz. There are very good reasons to use multiple bands including coverage issues, operational issues, and close quarters spectrum management. Exactly how many simultaneous conversations can you have from a small parking lot or from a single vehicle before someone's receiver gets blanked when using a single band?
As some posters say everyday on this board technology is only a small part of the whole "interoperability" picture. You actually need to have responders want to talk to each other. Once that is established the planning and operation is easy. As many have posted in this thread and others, conventional analog is both easy and cheap.
As has been brought up there is no reason for both the LAPD and LAWA radios to not have the UCALL/UTAC channels programmed. The LAFD could be included easily as well by equipping commander's vehicles with analog only UHF radios. With good quality, low tier radios available for sub $600 price tags the entire fleet could be equipped and they might even be able to talk to LA County units in the border areas.