Taylor Swift is a young lady, still trying to figure out who she is and what kind of person she wants to be with. Honestly, whether or not the relationship lasts is not really a huge concern. I just chalk it up to young love and finding one's path to happiness.
As a current pop star, her persona is a carefully stage-managed production, with theatrical elements designed for the maximum expansion of her hyper-celebrity status. On top of this, Taylor's persona has been defined by romantic breakups and the resulting songwriting these relationships generate. The prospects for a lasting relationship with Tom Hiddleston are slim.
Taylor Swift is an entertainer famous for her combustible relationships and the songs she writes about them. She is young, rich, and worshipped by her fans. Thus, she is under no external pressure to settle down, and only needs to extend a relationship as long as it's enjoyable. Both Swift and Hiddleston are busy with careers that keep them on the road, a difficult hurdle for any relationship. Ultimately, more relationships fail than last, and there is no reason to think this will be an exception.
Taylor Swift is notorious for failed relationships. She is a songwriter who wears her heart on her sleeve. Things never end up working out for her, and she winds up writing a song about it. This has been her past. Maybe someday she will find her soulmate, but until then, we'll keep listening to songs about her exes.