Is it really just nostalgia?

Yes, and no. People have different tastes in music of course, but I do think music in general was of more substance and of an overall higher standard in the past. I think the overwhelming majority of people would say that "Strawberry Fields Forever" by The Beatles is a better song than anything released by the likes of Lil Peep or XXXTentacion, for instance. You don't just have to be a fan of rock music to recognise the difference in quality between the two releases. There are thousands of people in their teens across the globe who are fans of 'classic' artists and bands such as Queen, Aretha Franklin, Pink Floyd etc. I do feel as though there is some merit in people saying that music was better in the past.

Nostalgia does play a huge role in it too though. I know for me personally, the music of the Late 2000s is hugely nostalgic for me (whether it be rock, pop, or rap) considering the fact that I was 8-10 years old during the era. I realise that songs such as "My Life Would Suck Without You", "Untouched" and "Gives You Hell" really aren't that great in the grand scheme of things, but listening to them instantly brings back great memories I have of being a kid in the Late 2000s and because of that, I perceive them through rose-tinted glasses. Music from the Early 2010s doesn't have that same sentimental feeling for me because I was in my tweens/early teens during the era and starting to go through puberty. Someone younger than me would feel differently about them though, just as someone older wouldn't feel the same way as I do over the Late 2000s. 

There were bad songs in the past, good songs in the present. Bad years in general for the charts in the past (1960, 1974, 1980, 1986 in my opinion), as well as great ones in the last ten years (2011, 2013, 2015). It's all a combination of factors and tastes honestly, not just a black-and-white "it's bad cause it's recent and good because of nostalgia".