Simpler lifeforms have been one of the most persistent and successful lifeforms on Earth, with a way longer existence than mammals. In fact, some theories postulate they are capable of interplanetary colonization, something we still cannot do!
You're right. I wanted to make a point how it's possible to have an objective comparison between life forms, but I judged simple life forms too quickly. I still think there are some obstacles simple life forms won't be able to overcome, but it's not obvious (to me) which of these have a better chance of surviving long term.