Mode A is pretty useless. Especially if one takes the charitable view that the driver behind all this is CAS busts; Mode A doesn't help with busts from underneath.
In fact, if there was the slightest possibility of Mode C being acceptable (rather than S) upgrading a Mode A installation to Mode C is relatively cheap. One can pick up a Mode C on U.S. Ebay (or from various secondhand avionics shops) for a few hundred quid, and currently there are plenty around due to IFR pilots having to put in Mode S.