There are different orbit levels in space. Higher orbits can stay there for hundreds to thousands of years, with lower orbits needing frequent boosts due to the small amount of atmospheric drag left. Lots, if not most of our satellites are in the higher orbits to reduce fuel needs. These orbits allow space junk to be deadly for a long time.

The thing is, there are also elliptical orbits which cause debris to be in a high orbit for most of its life, but then dip into the lower orbit for a period of time. Not only can these stay in orbit much longer, but they can hit the lower objects that need boosting like the ISS, thus being a danger to everything.

Once you have shit start exploding in all different directions, you get all kinds of orbits many of which will be destructive for many many years. Some of the debris will return to earth, but there is already enough right now just orbiting for years and years that we have to track. Not great.