Canada Goldenrod has been proven to have an allelopathic effect on some plants, and many studies have been done relative to that. Where I live, the next thing in the successional line to take it over will usually be blackberries, which have no trouble at all wiping out the goldenrod. If blackberries can get started in a spot where early successional trees are slow to pop up, they will form an impenetrable thicket that will completely stop any tree. When this happens, the patch has to slowly be taken over from the outside by trees at the edge of the patch. This can take many years in some cases, and if you stopped the trees from creeping in on the outside, the blackberries would never go away - and you would have a blackberry monoculture from now on.
Goldenrod is a welcome addition to my prairie. It fits in nicely between the NWSG clumps, and gets tall enough to provide good fall and winter cover (which is my primary goal). Another benefit of goldenrod is that deer feed on the basal leaves after the first frost. So, it is a food and cover plant. It can easily be knocked back with 24D if it gets too aggressive in some spots. I've done that occasionally at the edges of plots. When it dies, I will get a flush of annuals out of the seedbank - usually partridge pea and common ragweed will be the most prominent ones. The ragweed is native and the partridge pea was planted.