Ground elder is a xxxxxx. And there is a thing called Alexanders which is a nuisance in some areas.

But in general, you are right, we have about 2000 introduced plant species, and very few are a nuisance, and very few other plants have become extinct as a consequence of introductions. Local botanical ecological niches don't seem to be very crowded (when man doesn't utterly destroy it), there seems to be plenty of room for more in it. Of course if man is totally destroying the plant habitats, it can be different. There is very little native vegetation left in many countries like Chile and Tahiti and Madagascar and large parts of USA and Canada, but this is because man destroyed it, not because of displacement by introductions.

Introduced animals are much more frequently a problem - the zoological niche is more crowded and in-comers more frequently displace the locals without man's further assistance.

Britain used to have native rhododendrons before the ice ages. Unfortunately those kinds are now globally extinct. And we had all sorts of wonderful animals like hippos and elephants and hyenas and things.