The problem is defining what we mean by "culture." How do you measure the totality of a culture against any given year in the past? There is no doubt that television (entertainment, at least, if not news) is better today than ever before, both in terms of quantity and quality. But movies are a mixed bag, technically superior but too often script deficient. Books seem better but only because the sheer volume of them includes greater raw numbers of good ones, but not a greater percentage. The internet is great, but since it didn't exist in the past, it's rather hard to count it as an improvement or a decline. About the best we can say is that popular culture today is better attuned to today's problems and challenges that old popular culture is, since old popular culture was made for its own time. This makes today's product seem better by virtue of matching our current cultural biases and assumptions.
In the broader sense, some aspects of our larger culture are better and some are worse. Our politics is more disfunctional than it was in the 1950s, but less than in the 1850s. Certainly manner and morals have declined significantly since the Victorians, but are still better than in the colonial era. America is less powerful than it was even fifteen years ago, but it is more powerful than it was in 1890. Western Civilization reached the apogee of its power and influence around 1900 when it ruled half the world. Anything after that is a decline of sorts.
The question, then, is what aspects of culture are we using to argue for the superiority of one time over another? And for whom? If you are a multimillionaire, there was never a better time to be rich than in the Gilded Age, but if you are poor, then today is the best you've ever had it and tomorrow will probably be better. The middle class were most secure in the middle twentieth century, but they have more stuff now than ever before.
So, ultimately, the idea of decline has to be measured according to some kind of criteria. In the past I have been criticized for suggesting that elite (high) culture has declined since 1900 by those who took elite culture and political power as synonymous with popular culture or technological improvement. We can argue until we are blue in the face whether "culture" is better or worse, but unless we define what aspects, for which social groups, and what time periods, such subjective judgments are rather meaningless.