I don't find "argument via capitalism" to be valid in the slightest.
The statement "It's ok to do this because otherwise capitalist enterprise would collapse" isn't at all valid. It's an argument against capitalism, not an argument for the moral validity of the activity in question.
It's certainly an area that's ripe for serious philosophical debate. However, if the capitalist end weren't there, the products wouldn't exist. Farmers are never required to use a single Monsanto seed, ever. It's not like the world food supply would collapse without them, though there would be somewhat less food and it would therefore be more expensive. If you want a world with no Monsanto-style agricultural results, that's a totally defensible position. I personally don't mind living in a world with Monsanto in the market, particularly because they don't have any sort of a monopoly or oligopoly on seed, just their special deluxe seeds.
There are some more extractive elements to Monsanto's business, specifically their pesticide/herbicide and pesticide/herbicide resistance lines of products. Farmers aren't required to buy those either, and Monsanto may be riding a short wave with them anyways. What Monsanto has not done, but a lot of people seem to imagine they have, is to corner the market for seeds, make no genetic changes other than to make them non-reproductive, and then make only the modified seeds available in perpetuity to extract massive rents.
I personally like publicly funded research for agriculture (in Minnesota, where I live, our state university has been incredibly successful in this field and it has amply paid back the investments). It's not much cheaper, but it's not as rapacious. The Monsanto bargain doesn't seem like a good one to me, and if I owned a farm I doubt I would take it. But for those who would, it doesn't seem fundamentally unreasonable to me that a person can freely choose to engage in a highly restrictive deal to access the fruit of a billion dollars' worth of research. Outside of their designer lines their deals aren't so oppressive, and they aren't the only vendor around.