Why are new LEGO bricks softer than old ones?
Some of it may be your own perception changing, such as a room you remember being big when you were a kid, but which you find small as an adult. So when you perceive bricks as being softer, it could actually be that they aren't, but that your perception changed. (If you were to walk barefoot on LEGO bricks for one hour per day, your feet would eventually harden enough that you wouldn't feel it anymore.)

However, some may actually hold true, especially for weapons. Swords that are produced nowadays are indeed not as hard as they were back then, and that is simply for safety reasons, as safety controls put in place become harder with times. In extreme cases, they'll even use rubber, for example for the tip of the big spike introduced with exo-force. Another more subtle example is the circular saw blade: ever thought how dumb it was that they didn't use a cross axle hole in that? Wonder no more: it's just so that kids wouldn't put it on a motor (I know I would have as a kid).

As for the perceived quality changes, well, it's a bit of the same, there's a lot of it which is just that, perception; but there are of course real quality issues that pop up from time to time, which LEGO usually assesses the best they can. One key factor here is that there's a huge stock in their factories, so any changes they make can take a while before they appear in released sets. For example, they have switched at some point from buying precolored plastic granulate to adding the color themselves to white granulate, but at some point they didn't add enough color, resulting in translucent and more fragile bricks. They fixed that, but a lot of the already produced bricks made it into sets.

Also, LEGO can only learn about quality issue when customers actually complain about them, so when facing a real quality issue, it's always best to contact the customer service and explain the problem. Not only will they usually be able to provide replacements for you, but this will eventually reach the production quality assurance.