Monsanto holds the patent for Roundup resistant plants, which is why they can completely control all of the seeds. If you're not using chemicals or that GMO variant, then you don't need Monsanto at all.

They sell a lot more than just their round up ready seeds, they also have a lot of GMO for things like drought tolerance and some that can release a pesticide like protein. They also own/supply a lot of the organic/heirloom subsidiaries that people believe to be competition.

It's actually pretty hard to avoid using a product from Monsanto, there's actually websites that have list you can cross reference to avoid it if possible.