What is the advantage of self-pollination? Why is cross-pollination still considered more progressive

What is the advantage of self-pollination? Why is cross-pollination still considered more progressive than self-pollination?

Self-pollination is beneficial, first of all, because a plant can grow alone and still continue the genus, one does not need another plant to be able to reproduce. This is beneficial if resources are scarce or there are other obstacles to the growth of a group of plants.

Cross-pollination is more progressive, as gene heterozygosity and genetic diversity are preserved. Since constant self-pollination leads to the transition of most genes to a homozygous state and degradation of the species, cross-pollination is preferable.