This. Alot of drone guys or people who dream of drones forget that there's GA planes up there. I've had to dodge stupid cameras on balloons while landing. That's so dangerous
ADS-B is effectively to replace Mode C. There's nothing in the mandate that expands Mode C required airspace. So that leaves a metric ton of space not covered by the rule for conventional airplanes, let alone drones. Drones of course need a totally different and scalable system.
Something possibly like a centralized flight plan, that's given an ack/nack/patch, and then the drone flies that plan on its own without needing constant comm like today's aircraft do (more like IFR plans where you're expected to complete the entire cleared flight plan, to the minute, in a comm failure). And then all drones share all flight plans that affect their routing. And all nearby drones rat each other out if any drone deviates from their flight plane. That'll scale. ADS-B may not.
1. For ADS-B out, you, of course, need all the requisite sensors (GPS, etc).
2. The correctness of ADS-B data protects human life. Naturally there will be much stricter requirements (certification of reliability) on something that produces this data and introduces it to the system, than for a "just for fun" receiver.
ADS-B is already mandated for 2020. The spec is cast in stone, and the only thing that could change is the month it's enforced.
As we've seen with the SoCal fires last week, integrated tracking is essential for safe low-level flight.
Certified ADS-B avionics prices have ranged from $10,000 - $1,000,000+. Experimental prices are around $2,000 now.
Obviously that's more than the price of most consumer drones.
Also, what load can the ADS-B system handle? How many drones would overload it?