My theory on this is that quality may have slipped a little recently due to cars being built to a cost. However, many cars today are much cheaper then their previous counterparts. Mechanical longetivity on engines and/or transmissions are probably still the same.
Suppose that car x was built to a cost, and the manufacturer manged to knock $10,000 of the cost of the car. The car then ends up in the dealer 6 times during the warranty period to fix defects. Warrenty expires and car takes three more trips to the dealer to fix defects, total cost $5,000. Car is know good as old car for still a cheaper price. That's my theroy on it. It all adds up in the end, and the current car with all its gadgets is still cheaper than the more basic, previous model.
Also, a lot of the defects are found in electronic gadgets that weren;t even present in the previous cars. So even if present reliability if each individual component were the same, it would still suffer more problems because there is more to go wrong.