Here is what this video says, in a nutshell.
"Hey, you guys who never got to do anything in Vanilla because you didn't have the time/inclination, you are depriving yourselves of an experience that you never would have had anyway by doing content that you can now do!"
The "journey" he describes is neither the goal of everyone, nor fun for everyone. It is a constant theme in the complaints that wow is "dumbed down", but it is one without objective basis. I fucking hate "the journey". It was long, tedious and boring. If I wanted to play a game with a long time commitment that requires skill, I'd practice and get better at chess. Comparing WoW right now to WoW in Vanilla/BC, I have SHITLOADS more to do. Every day, I can log in and do something productive with my time. When it was kara days, I would log in once a week to do kara, knowing that I already had the gear I wanted out of there and I was never going to go on and do the harder stuff anyway at the time.
"How is this fun?"
How is the old system fun? The answer is, to different people, they aren't. However, the majority of the user base isn't in the 1% that got to do naxx original, or who would EVER have done naxx original or its counterpart today if the system hadn't changed. People call it instant gratification as if that is a curse word, but it's not. There is absolutely nothing wrong with a model of game that offers both instant gratification and long term development of a character. It's great. It works.
The guy says in the video "end game raiders are a dieing breed because new players have a lack of motivators, by way of hard content. Hard content is a good thing (which he says emphatically)". He is wrong - or perhaps more accurately, he is right, but his implication is wrong. Motivation exists in two forms; whips and candy. The old method used whips. People didn't want to be there - they just had to be. Now people who are end game raiders are people who want to be end game raiders, and as a result, there are perhaps far fewer. This is because - and I can't state this emphatically enough - most people do not want to put in the commitment it takes to be a succesful endgame raider. There are very, very, VERY few endgame raiders that are succesful at pushing to the end of each tier while it is still current that don't raid something like 10-12 hours a week at a minimum, as well as overheads for consumables and reps and so forth. This is all well and good, but the vast majority of the world does not consist of people for whom this is a desirable life choice.
The problem is not "there are now less endgame raiders and this is a bad thing". The problem is that people are taking the past, putting it up as the yardstick, and measuring compared to today. In fact, the past was where people were compelled to be and do what they don't want, and now are free to do the content in a socially appropriate way for them.
PS: My god this guy has rose coloured glasses.