I like the idea of nursery being more focused on child-led exploration and developing social skills and personal care skills. I suppose it depends on your MIL and I would never think it was "disgusting" for someone to not send their child to nursery. But frankly you couldn't pay me to leave my kids with either my mum or MIL on a routine basis while I worked. It would be so unstimulating (I'm pretty sure there would be a lot of screen time and they would be fed a lot of rubbish and never leave the house). Occasional babysitting is absolutely fine, but on a regular weekly basis, no way.
At nursery, my daughter got to do so many more things she never would have done at home with one of them: cooking and baking, forest school, there's a mud kitchen for mud play, they learned about fire safety and the fire engine comes once a year, Christmas plays, just all sorts of really creative, organised yet child-led activities that my mum/MIL certainly never would have done. Plus I liked the EYFS assessments and such that they do as we could see how she was developing and that made for a smoother transition to school as the nursery and school just liaise with each other about all of that.
But really, the most important bit was the social skills and personal care side of nursery. We live really rurally and there are no toddler groups really or anything like that around here (you know, even if I didn't have to work). So my dd would have had almost no interaction with other children. At nursery though, they put a lot of emphasis on sharing and developing friendships and conflict resolution, etc. They cook together, share communal meals, do chores together, etc. It was a really beneficial social experience. Plus, they encouraged her to be really good about handwashing and taking care of her things and using the toilet independently, etc. I think because she was so used to already doing all those things for herself (we try to encourage it, but realistically, she can't easily reach other places so we have to help her a lot), it was much easier for her to do it when she started school. Also, she made several friends who have now gone to the same school so it made it a bit easier too.
I don't think it's 'bad' not to go to nursery. Realistically, it's expensive. We paid as much for nursery as our rent each month. It may not be an option for everyone financially. In our case, we have no family help, so it was our only option. But even if I'd had the choice, I'd still have opted for nursery as I think it was such a positive experience, much more so than anything she could have gotten at home with family (I stayed home with grandparents when I was little, I don't think it was the best choice for me either, so that is definitely a factor too).