I don't think your concerns would be addressed by a longer beta as much as a rework of what you don't think is working right; more to do at endgame and class restructuring are major projects that would push the game launch back who know how many months.
I think that complaints and frustration over class design fall mainly in the realm of the forum goer, the hardcore player, etc. The player base at large doesn't really feel the impact of the things that frustrate the player pushing more challenging content, and that is where the money is at for Blizzard. And I don't mean to imply that I think Blizzard doesn't care about the concerns of the hardcore crowd, especially because the race for world first and the meta involved in doing high keys and arena drives a lot of chatter as well, but there is a line where you have to decide if something is good enough, i.e., perfect is the enemy of good.
Now for content, there are perhaps easier fixes, and ones that would affect a broader swath of players, but I'm not sure what else they could add beyond alt leveling, pvp, Mythics, raids, pet battles, the secondary systems of the expac, Torghast...I mean, I get it, some people will overlook various pieces, and feel bored because they have nothing left, but as with many things, how big a percentage of the player base does that equate to?
Lastly, while I am rarely cynical about Blizzard's marketing approach, I can see where they want to drive as many people to that initial buy as possible; 2 million people that buy the base edition of SL, that only stay subbed for two months and don't play again for a long time, or ever, still generated $140 million in revenue. Which, if I'm being honest, doesn't solve everything; if you take the games core for granted, eventually it will erode and you lose sustainability after that initial push, and we've seen that during the life cycle of Wow, as it approaches 16 years old, that the last few expansions have lost more old players than gained new ones. The fact that it is still relevant in the gaming world is impressive, but maintaining it is a challenge.