I tell my children not to hate (it drives them mad). This is a good example of why hating is bad.

First, do you mean all flies? Or just the ones that chew lumps out of your skin in the summer? Or only the ones that crawl on your Danish pastry while you are putting the kettle on for elevenses? Or the ones that lay eggs on meat in the butcher's shop?

Perhaps you mean the many millions of flies that get eaten by swallows, house martins and bats on the wing, without which they'd probably die? Or the millions more that are eaten off bushes by warblers and blue tits?

Maybe you hate the maggots that eat decaying flesh (rather usefully in medicine) and speed its recycling? Or the flies that pollinate flowers - many plants prefer specific flies to the bees and butterflies we are more likely to recognise as pollinators?

Please be specific :)

Pete has put it very well. I assure you that the impact on ecosystems would be drastic. We would lose untold treasures and valuable resources.  Also, what kind of flies? You could go to the nearest garden or farm and find flies from a millimetre long to a cm or more. Flies that eat caterpillars, dung, flies, ants, carrots, weeds, fruit, living flesh, dead flesh...

Let's assume you mean house flies and bush flies? Do you hate them while they are tickling you, or while they are recycling your refuse into valuable compost, or feeding your chameleons and wagtails?