Think the number of units out there is exactly the sort of technical information one needs to decide which EC to go for.
I think there is probably more to this kind of decision than pure arithmetic, although your point is undoubtedly valid, so referring back to the nub of your question, let me ask:-
What number of units would be the tipping point for you to go for PilotAware ?
What was the criteria that brought you to that specific figure ?
a) If PAW did ADS-B out (even with SIL=0) no critical number as it would allow me to get most of what I want (FLARM without the need to use ground stations would be a bonus)
b) I don't have a preconceived number in my head, see above, but think that half of the GA spectrum would be a good starting point.
Why have I not bought a PAW yet:
1. I think it is as important to be able to be 'seen' is as important to be able to 'see`
2. therefore units that transmit only on the PAW frequency are maybe not a problem for me, as I have a TXPDR, but the principle of having a frequency which is not used by other EC is not right (my personal view)
3. The initial units were kits which were well beyond my technical ability - maybe not the physical sticking together but 'programming' side certainly seemed to be that way
4. The 'spread' across the GA spectrum appears to be fairly limited and mainly across the UK; add to that the notion from various threads that a fair few folks had bought the kit for it be relinquished to the drawer after a few flights with it.
5. A fair bit of my flying is outside the UK and in environments were FLARM is widespread in use; never yet seen a PAW in 'the wild'