No, mass starvation would not ensue from having to fight weeds using mechanical means. It would take more work and more fuel, but it is eminently doable if the need is there. Especially if the change would be gradual.

Making do without artificial fertilizer would be a lot harder.

I don't think that is the only alternative. If the end goal is to preserve life for humans, completely nuking the soil into a wasteland, treating it with carcinogens and then allowing a company to genetically modify seeds and copyright them is a pretty bad and short sighted strategy.

Allowing a known carcinogen to make crops "easier to harvest" has to do with profit margin not food supply. People literally use this to kill dandelions in their yards. I have known many people who have died from cancer. I have eaten dandelions, while bitter, are actually healthy. A good start would be to work with nature instead of trying to out engineer it.

If roundup is your alternative to starvation you're probably just delaying the inevitable.