Over time ADSB transponders will get cheaper and more common as the Mode C transponders age and need to be replaced.
At the last transponder check the Avionics LAME said it would be the last time the old Mode C unit would be able to pass the tests.
I doubt anyone will buy a new Mode C unit now and by natural attrition and default ADSB will become the norm.
In my case it looks like the Trigg 31 will fit directly into the tray where the old Narco AT150 sits now. The Trigg transponder, EO and fitting should cost less than $5k. Spending $5k is a pain, but is not a show stopper. Instead of spending $3k to replace the old Mode C unit it is only about $2k extra for an ADSB upgrade.
Everyone will use GPS and ADSB. Radio-based navaids and Mode C transponders will end up like VHS VCRs - just a memory.