A couple of thoughts on longevity and in general.
For Longevity, like Lidstrom and Bourque, it is my opinion that there are two factors at play in the development of a player, talent/skill and experience. Often times, a player will decline in skill and improve in experience allowing him to remain effective. The rate of decline is important as certain styles tend to reduce the impact. A style like Lidstrom or Bourque with low impact, efficient play styles will of course let their skill remain high over a long time as their experience lets them compensate, perhaps improve, later in life.
In general, a key point, to me, was the expansion in 67. More teams means more players, means more exposure to youth, more grass root programs, more television deals etc. In general, people started watching hockey more often and in more places. Combine this with advances in television proliferation and more kids could watch and emulate their idols. By the time the mid 80s rolled around, a whole generation grew up being able to live hockey all the time and globalization added a new dimension of international competition. This fundamentally changed the talent level in the NHL, there was more demand for good players and more supply from around the world. Therefore, players became a lot better than they used to.
Now fast forward to the new millennium and the internet is show people around the world everything they could want from hockey. Watching tricks and skill displays on Youtube or watching games over and over on DVR combine with even more teams than ever and even more rampant global exposure and adoption of technology and I am going to guess that in about 10 years, players will be even better than today but guys like Crosby will continue to impress and earn mid-late career accolades like Lidstrom has.
So overall, players today are better than before from a skill point, training point, health point, etc. As much as Gretzky and Orr dominated the competition in their times, they simply would not be as good today unless they were born and raised in our new society with our technology etc.