It's hard to imagine a total block of access to orbit. More expensive and inconvenient, yes. You can always go for polar orbits, and higher orbits. (Or lower orbits - your satellite will deorbit in a few months/years because of atmospheric drag, but the same goes for any junk there.
It makes a difference where the junk is. As you say low orbits clean themselves up slowly, but geosynchronous Kessler syndrome could last thousands of years, and geostationary orbits are pretty essential.
Yes, if the geostationary orbits were inaccessible, you would require maybe tens or hundreds of satellites launched in a ring to achieve the same the same result.
:-/
On the other hand geostationary orbits or so far out it's maybe harder to achieve a dense enough junk field out there to matter? I have no intuition about such scales...
