environmental weeds

My neighbour recently told me that lots of her friends in the late 50th are dying of cancer, and she says that it is due to the spraying against weeds here, mainly blackberry. (They are spraying something else meanwhile)
I always has my doubts that you can help nature with weedkillers, and so many years of weeding bushcare and spraying, I can't see that the weeds are gone or eradicated which is the aim of all these actions.
There is a fundamental law in biology that species procreate, given favourable circumstances, they might procreate very much taking over whole areas until the population breaks down. This happens all the time (with mankind as well).
What would happen if we would leave nature dealing with these introduced  species on its own?

There is a second very funny thing about council information and weeds: they never tell you if a weed is useful. I.e foeniculum vulgare
they tell you that this is a plant which is very similar to the edible garden fennel! Why shouldn't we use plants which are abundant and a classified weed?

I would say that most herbicide use is made out of an 'economic' decision.  It is cheap and requires no skill to turn vegetation into bare ground using a spray.  The weird thing is the seeking bare ground.

The US noxious weed lists are often related to livestock toxicity.

By contrast, English Ivy is spreading rapidy, seed carried by fruit eating birds.  Some foreste sites that have had Hedera helix for a couple generations are overgrown with no forest regeneration, and massive windfall from ivy covered boughs.  It is shade tolerant and no natural predators.  It will spread into old growth Spruce forest in the Olympia Park, replacing diverse groundstory vegetation with a monoculture.  The nursery industry fights to keep it off the noxious list, and sells thousands of starts to suburban homeowners expanding into the forest land.  It is very resistant to herbicide application due to a waxy cuticle.

Some folks in the Ag community still sell Reed Canarygrass... Phalaris arundicaceae was sown for erosion control, and now replaces complex native ground vegetation in swampland.

On the otherhand... many "invasive" plants invade land that we cut, scrape and leave for dead -- signalling the return of live to devestated ecosystems.

Cancer by contrast is in the very life blood of modern industrial society - modern weed killers possibly being the least of it compared the deluge of Ethyl methyl death we manufacture.

I don't think this is a simple issue... but rather one full of unpleasant tradeoffs, deeply rooted.

Don't kill the messenger..

It's not simple. Sure you don't want to see old growth forest killed or bushland full of cotoneaster. And maybe mankind plays a role selling plants in nurseries. Unfortunately most of these weeds are "ornamental" and of little use. They are mostly not ornamental because they are pretty (i.e cotoneaster or privet), they are only classified as ornamentals because they are not useful. Nurseries like these type of plants because they are weedy, easy to propagate and cheap to sell.
Does the old growth forest die because of the ivy or are there other reasons as well?
A better and cheaper idea would be to search uses for these weeds, publish them and people would harvest them. (if aphrodisiac properties could be attached to a weed, it would certainly be eradicated)
Glyphosphate is harmful to the environment too, I think for frogs.
The only method, which I can really accept is to scrape each weed at the right time of the year and use one to three drops of Glyphosphate. It works actually much better than spraying.