Many sows develop cysts as they get older, but the majority of them is not causing problems. There are also now alternative hormone treatments available to a spay (chorulon). The number of forum sows that actually require a spay because they develop a womb or or ovarian cyst problems is much less than 1 in 10. The forum has been going for 10 years and thousands of piggies have passed through this forum in that time.
Weigh that up against the cost and the operation risk, especially in the hands of a vet who is not very experienced in guinea pig/small furries operations, and you will understand why most piggy owners decide against spaying their sows preventatively unless they have access to a very experienced operating vet. The situation is not comparable to rabbits where the cancer risk in unspayed does is indeed extremely high.
You are not a bad owner at all - or we are all bad owners on this forum here...