You're just not getting it. Transponders aren't required to be fitted to VFR aircraft operating in G.
And as I said above just making an imaginary line in the sky saying you must have a transponder here, does not stop a transponder-less aircraft from operating there, so there is really no less chance of encountering a rogue transponder-less flying machine in such airspace. The lower you make that threshold the more likely this will happen as those aircraft that are not fitted wander around oblivious to the rule are more contained to low level flight than roaming at say above 5000 feet. You could say all aircraft must have a transponder above 5000 ft and within 15nm of RPT CTAF would really do the job, regardless of airspace. Without primary radar ATS would have no clue if an aircraft without a transponder is in class E or not. Now there are mistakes, but anyone who flies around the wilds of Australia knows there are many pilots that are clueless or just flaunt the rules like flying VFR through cloud or in less than VMC.
Now when I say I like flying around in class G I refer to class G above say 8,000 feet because the likelihood of meeting random aircraft expands rapidly the lower you go past 5000 ft AGL. So above 10,000 all aircraft are required to have a transponder, and while the occasional VFR TBM crosses your path it's really empty airspace and you get timely warning of anything in your area from ATS without needing a clearance.