Are there really a lot of concerns? Above FL180, you’re likely going to be flying something pressurized. Sure you have to deal with a rapid decompression and emergency descent, but that takes minutes.
Not necessarily. Plenty of turbocharged piston aircraft that can go to FL250, some even higher.
Any pressurized single or twin piston is pressurized to 8K, right?
No. Every pressurized plane has a PSID limit for its pressurization system. The higher the PSID, the higher you can fly with the same cabin, or the lower the cabin can be at the same altitude. Some are better than others! But some of the smaller planes can outclimb their pressurization systems such that you may want/need to put on an oxygen mask even though you're in a pressurized plane! Here's a few examples:
Cessna P210: 3.37 psid, 10,000 foot cabin at FL200
Beech P-Baron: 3.9 psid, 10,000 foot cabin at FL220
Cessna 340 and 400 series twins: 4.2 psid, 10,000 foot cabin at FL230
Piper Malibu: 5.5 psid, 10,000 foot cabin at FL280
TBM: 6.2 psid, 10,000 foot cabin at FL310
Citation CJ4: 8.8 psid, 8,000 foot cabin at FL450
Boeing 787: 9.4 psid, 6,000 foot cabin at FL430
Citation X: 9.7 psid, 7,000 foot cabin at FL510
Concorde: 11 psid, 5,500 foot cabin at FL600
I think the they should replace the 3rd class with Basic Med. There is nothing in the 3rd class medical exam that will catch/indicate issues with flying in the Flight Levels. Not even in the 2st class.
Over 6,000 pounds? So a 6,000 pound airplane is OK, but 6,001 will fall out of the sky if you don't have an AME examine you? The same for the speed restriction. And even the number of seats.
With the FAA, it's all about who you can kill. The more seats, the faster, and the heavier the airplane, the more likely that you'll kill more people - Either pax, or those on the ground - if and when you crash due to incapacitation.
Whether it's pilot certificates or airplanes, the higher the potential for hurting or killing more people, the higher the standard of care in who gets to fly those flights, what medical exams they require, what equipment is required on the aircraft, etc... Yes, 6,000 pounds is a fairly arbitrary number but they have to draw a line somewhere and that seems like a fairly reasonable place to do it. You can fly a 310 or Baron, but not a King Air or PC12.