The End of Trek
I think it is fair to claim that Trek's Golden Age comprised the TOS movies as well as early and middle TNG. TOS is often quite raw and unsophisticated and while we all have our favourite post-TNG series none of them are as good as TNG if we try to be honest and objective.
I think there is one key structural reason that explains this. In the beginning the universe was empty and in the end it was crowded, universe meaning not just the galaxy, familiar aliens, sense or absence of mystery and so on but also the social universe which became more soapish, emphasizing personal relations among the main characters more and more.
Let's take a look at the Klingons. Initially there are many variations, the cold warriors from TOS, the barbarians from TSFS and early TNG and the more human Klingons from TUC. This was interesting but also a bit unpolished. When TNG changed the Klingons into the Viking-like culture and began to tell epic stories with Sins of the Fathers they became undoubtedly better. But then Trek was flooded with Klingon stories and while they are in no way bad (I do not belong to the 'there are too many Klingons in Trek' fraction) it all became a bit stale. DS9 added little new to them and VOY's B'Elanna did neither.
Of course the crucial question is whether ENT's and ST09's going back to the roots, i.e. varying Klingons again, restarts the cycle.
Now the Klingons are just one example of this overall trend from a blank slate towards a crowded table, from individual stories to serialization, from exploration towards epicness.
To get back to the crucial question, I think that the attempts from the last decade to balance these elements, be it via innovating old aliens, balancing serialized and individual stories or rebooting the franchise itself, are too conservative and remain in this deadlock. There needs to be a more radical cut.