It's easy in some respects with one, but then, in others it's trickier. No more than families with more than one I guess.
I would have loved more but we had just the one. I remember feeling a lot of guilt about that for a long time, but then I was at the funeral of a family member. That person had been an only child and the turn out was bigger than I've ever seen... even for Irish funerals. Grown men heartbroken at the death of one of their oldest and closest friend. And that actually comforted me a lot about leaving just one behind when I die. I knew that DS would be as lonely as he chooses to be.
I made a bigger effort to do lots of time with his cousins on both sides. My lovely SIL did afterschool for about 3 years for us so DS was like the baby of their family in many respects, and their house is treated like ours by DS. We are only a couple of doors away from their house so they are in and out to each other all the time. I try to go as much as I can to the cousins on our side as well as we are fairly close knit and I hope that carries through to adulthood for DS.
He gets bored sometimes like any kid, but the flip side is that he's well able to find things to do himself and is quite a social and sociable kid. I ensured he's got bunk beds so offered lots of sleepovers, and offered loads of play dates - I don't care if they weren't reciprocated. We deliberately moved to somewhere where he could safely go out to play with the neighbour kids so on a fine day he's never indoors. He has wished for a sibling from time to time, of course, but it is what it is.
Holidays are fine, we've never put him in a kids club. He wouldn't have wanted to be left (again with a sibling that might have been easier?) DH and I can't lie by a hotel pool, never could anyway so DS comes with us on boat trips or snorkelling on the beach like we used to do as a couple.
I'm not worried about when he's grown and gone. I came from a big family, so getting peace and quiet was a novelty and I relished it when I did. I moved out at 18 often living entirely alone and I was never bothered. Even now when the house is empty for a weekend, I've a list of things to watch or sew or read and I don't get lonely. I would happily do a solo holiday for example. I might feel differently if my health becomes an issue but I'll figure that out when I have to!
And then there's his inheritance. He will never need to worry about affording his own home or have to split it with siblings. He'll get our house. That's something my neighbour worries about for her 3 kids - that even with an inheritance and a partner to share a mortgage, that house prices might be out of their reach.