such constellations are in LEO, which means their orbits decay in years, not centuries. The satellites associated with "space junk" are in higher orbits like geostationary.
Geostationary satellites are way too far and few in between to meaningfully present a problem. The majority of dangerous (in Kessler syndrome sense) junk is on higher LEO and eccentric orbits.
Correct. Most non-Starlink constellations LEO are going up around 800 - 1200 km altitudes. Those orbits have century to millenium level deorbit times and pose significant Kessler risk.