When a garden of herbaceous perennials gets any bigger than a four foot wide, easily maintained border, it is an invitation to chaos. Anything that self sows will, often abundantly. Clumps expand. Large plants gain considerable size and take more space. There are winners and losers in all that competition.
The original design will be lost in three to four years without considerable effort to keep it intact.
Now add in sweet bunnies, groundhogs and horses two feet in along one fence line and another level of competition is selecting for winners and losers. I know for a fact that the first item to go in a very diverse buffet is always the purple coneflower. The varmints won't be getting any colds and flu, not in my gardens.
But abundance wins in the end. There is plenty for predation and enough left over to put on a floral extravaganza. The gardener just won't win any high standards maintenance awards, especially since he lets some of the indigenous wild flowers join in on the show.