From the Wikipedia page you linked,
Aspens typically grow in environments that are otherwise dominated by coniferous tree species
Yes, aspens do tend to dominate their area, but during the time between when they're introduced to an area and they dominate it, there is at least some variety. Not all trees are as easy to dominate as others. One of the factors that helps them dominate an area is (same source)
the rhizomatic nature of their root systems. Most aspens grow in large clonal colonies, derived from a single seedling, and spread by means of root suckers
Each individual tree can live for 40–150 years above ground, but the root system of the colony is long-lived. In some cases, this is for thousands of years, sending up new trunks as the older trunks die off above ground.
That suggests to me the process they dominate a forest is by basically taking over the ground in a long, slow process, so it would take a while for them to fully dominate an area.
Looking at the pictures on that page, , we see an example of an aspen forest with at least one fir on the bottom right. I think there's some other non-aspens in that picture that are still within the body of the forest, but it's difficult for me to tell for certain.
Several decades ago I lived near an arboretum with an aspen grove, but they still had quite a few other trees around. In the roughly 14 years I was either living in that area or coming back to visit, the balance didn't seem to change much.