Will ICAO 24 bit addresses suffer the same fate of IPv4?
I don't think there will be an ADS-B mandate specifically for drones. Instead I expect ADS-B will be mandated for certain airspaces.

The US has taken this approach from the beginning, Australia has a combined airspace &amp; flight rule approach (IFR in class A,B,C &amp; E airspace). The EU so far has based the ADS-B mandate on aircraft weight (>5700 kg MTOM) or speed (>250 KTAS). That is likely to change in the future.

For drones operating outside this ADS-B mandatory airspace, no ADS-B will be necessary. I guess the number of drones inside mandatory airspace will be limited, since there is no efficient way to integrate them into the Air Traffic Control system. However, over time this will likely change. 

The big problem I see is not so much the limited address space, but the overloading of the 1090MHz frequency that ADS-B will be working on. On the long term I see a need to rethink frequency allocation for surveillance and navigation (and while we are at it, communication too). The limited number of addresses should be dealt with at the same time. With so many investments in the current system, it is going to be very difficult to transition to a new system.