Read Nothing Feels Good. All your answers lie within that book:

Nothing Feels Good: Punk Rock, Teenagers, and EMO https://www.amazon.com/dp/0312308639/ref=cm_sw_r_cp_apa_i_b1bqDb7MDE8FN

As a TL:DR, Emo has gone through several waves of popularity and genre fusions, so it gets confusing.

To summarize the last 30 years, Emo started as a reaction to political hardcore bands in the 80s (Rites of Spring), then it sort of mutated into alt/indie rock (Sunny Day Real Estate, The Promise Ring, Jawbreaker) in the 90s.

The reason people have the reaction to Emo they have now is because of its next mutation was less a musical style and more of a lifestyle/subculture that blew up in the early to mid-aughts. This was Emo's largest mutation, which is why it's hard to define.

Bands like Dashboard Confessional led the way, while bands like Brand New, Taking Back Sunday, and (whether they agree/like it or not) Thursday pushed it along for bands like My Chemical Romance to elevate it to its height.

Today, we're seeing a reaction to that mid-aughts Emo subculture. Beginning in about 2009/10, bands like Fireworks and The World is a Beautiful Place... have taken us sort of full circle. Now Emo can be defined by twinkly or clean guitars (Tiny Moving Parts), or even more of a grunge-y sound (Citizen, Movements, Moose Blood). For a full experience of what Emo is in 2019, listen to Sunsleeper; they are an amalgam of Citizen, Movements, and Brand New.

For me Emo is back in that sweet spot, like how it was in the late 90s, early aughts before it blew up. I don't think it's going to blow up that way again, but for now it's nice to have consistently good music coming out from the scene while everyone else ignores it.

I don't think you asked for a history lesson, but get me started on Emo and I can't shut up (I really just wanted to post the link to the book and look where it took me, haha).

Thank you for coming to my TED Talk, or whatever the kids say these days.