If leaves were kept indefinitely they'd also have to deal with damage, wear and tear and so on. Biological systems often seem to prefer replacing over repairing damaged parts, which is perhaps another reason why leaves are replaced. For instance our skin is constantly being replaced, so injured pieces are discarded instead of repaired.
Losing needles is not a disadvantage as much as an advantage in the most northern regions of the world where the old needles can still do photosynthesis when there's good light but it's still too cold outside to grow new leaves and get nutrients from a still frozen earth.
Note that a few coniferous trees like the larch are in fact deciduous.
Leaves were a 'major invention' and a 'big step up' in climates where they are the most appropriate thing. In some climates, coniferous trees still dominate because they are the most adapted species to that specific environment. Evolution works through what's the most apt at surviving in given conditions; dominance is conditional to the environment you frame it into.
Definitely not true; there are plenty of pine trees in Georgia (the one in the US), and not just in the (rather mild) mountains.
I will admit that there seem to be more pine trees the further north you go.
Leaves were indeed a major event in the the evolutionary history of plants, BUT, leaves are actually older than needles. Many primitive gymnosperms had leaves, as did primitive ferns. Needles are basically leaves that have rolled in on themselves.
Conifers are dominant over very large areas, they have not been pushed out of almost every niche in the world. They are the dominant group of plants in northern and southern boreal forests and cover a significant portion of the earth's surface.
And about leaves: we were talking trees - of course herbs existed hundreds of millions of years earlier.