
I have always felt exactly the same, so know where you're coming from. Indeed, some of my girls only had one litter before being spayed, for various reasons.
However, I do have an extremely good bitch that is probably the best that I have ever had. She had 2 litters, two pups in each, and I kept the first litter for myself. Her next litter was 2 boys, and I already had the male pup from her first litter, so I didn't keep a pup. The one boy went to a show home, so I would always have him in mind as a back up if needed.
However, when she had her last season, I realised that this was her last chance for breeding. I took her to have a check over, and my vet reckoned that, as she had such small litters, breeding had hardly affected her. She has very easy whelpings, and is a super mum. So I went for a 3rd litter for the very first time. She had 2 pups - which I kept! :-)
She is now spayed, but I have no regrets knowing that her last whelping was just as straightforward as her first two.