"Why is this popular?" Because it's a catchy, anthemic song that resonates with a lot of people. It's got a rather specific focus, and it apparently doesn't resonate with you. Does that make it a bad song? To you, surely. Objectively so, as you're characterizing it? ("If Taylor wasn't the 'IN' thing this surely would not be popular.") No -- as evidenced by the existence of people who like it and can tell you WHY they like it. Have you searched these people out? You're SURE they don't exist?
You have nothing against Taylor Swift? You are essentially calling her greedy, evidently because you haven't considered that a person can legitimately like such a song without being a trend-chasing fool. While I'm not crazy about the song, I have listened to its entirety, and it does NOT seem like an outright cash-grab to me. I believe there's a legitimate audience for the song, it's just that it doesn't include me. Doesn't make it an objectively bad song. No one's ridiculing me for not loving it; if they were, I would criticise them, not the song.
Why am I bothering to write this? Aren't you entitled to dislike a song? Sure you are. But, if you're going to dismiss another human being as money-hungry, and a large swath of people as a herd of fools with poor taste (since they would buy a song with "zero substance"), expect a challenge. You are NOT entitled to foist your negativity on the world without questioning as to the extent you've thought things through.
How's this different from what you've done? Aren't I just doing the same thing I say YOU'RE doing? No. I pointed out what you said, how you said it, and how it can be wrong. I'm not dismissing you; there's enough of that rubbish in the world. Challenge my ideas if you wish.
What could you have done differently? Aren't you entitled to dislike the song? Again, sure. But say something like "I didn't like the song. I don't like the melody or the lyrics; I feel like it lacks substance, and it just doesn't resonate with me. It sounds too much like a generic summer pop song to me. I say pass." There, you communicate your dislike, without unwarranted dismissal of the legitimate actions and feelings of others.