Too right SSD, They will charge us to install the equipment we dont need, charge us to certify that it works every year and then track us and charge us for using their ATC services that we dont need!
IO-540, the cost of the equipment is significant. There are lots of cheap one or two seaters owned by groups, where people even flying as little as 10 hours a year are saving compared to hiring club aircraft. some of these aircraft cost as little as £5000, so a mode S transponder is going to be 50% of the cost of the aircraft!!
There is no reason people couldnt be taught reasonable navigation in the 45 hr ppl, or even a 35 hr ppl, if it wasnt done at a large airfield with giant circuits. Then people might not take 20+ hours to go solo, but might go solo in 7 or 8, thus a lot more time left for Navigation! Hopefully if the NPPL gets more popular and can be done in permit/private cat aircraft from unlicenced airfields, then the rate of learning will be much quicker, just as it is in the microlight world.
The US does have a few places where Transponders are mandatory below the TCA, not sure if they are all 30 mile diameter though. However, there is an exemption for aircraft without electrical systems flying underneath the controlled airspace. The vast majority of US airspace can still be flown in without radio or transponder.