Well, as far as my understanding goes, basic building blocks are still necessary to life. For instance, amino acids.
As for other things, if something (even a single-celled organism) is thriving as it is, then it doesn't need to evolve, does it? Viruses and bacteria don't necessarily need to evolve into another life form, because they're fine the way they are. Although they do evolve to become more resistant to drugs, or to become infectious to new species.
With regression....why? What benefit would there be? Evolution is about benefiting whatever species you're talking about by slow changes over generations. If there is no benefit, then it shouldn't be taking place.
Why can't we force it to happen? Well, I would guess that it's simply because we don't know how yet.