I have worried about loneliness/building friendships, not so much due to being aromantic, but due to being a socially anxious adult who has moved around a lot over the years. It's hard to maintain friendships when moving from country to country every year. It was the main reason I chose to settle in the UK. For all I loved travelling, I wanted to be able to form some really close friendships and put down roots with them.
What I've learnt is that thinking about the future (what will happen when they get married, have kids etc.) is pointless, it just gets in the way of enjoying your relationships now. Some friends drift apart without big life events to get in the way, simply because everyone changes over time, that's just how life works. The same is true for romantic relationships. I wouldn't still be friends with the people I new at 20 even if I did still live in the same country as them, simply because at 30 I am a different person, with different interests and values. The friends I have now, who I have closer relationships with than I could have ever imagined 10 years ago, I wouldn't have wasted 10 minutes on when I was 20. Practicing Relationship Anarchy has taken away a lot of my concerns about being abandoned for a girl/boyfriend and the need define and overthink my relationships.
Make friends, enjoy their company, love them without expectation, and be prepared to let them go if that time ever comes.
On a more practical note, I made my friends in 2 ways, via Meetup.com (especially a meetup group for people with mental illness, there's no one more understanding of social anxiety than other people with social anxiety), and via munches.