It takes quite a special mind to fight for the protection of something that faces no threat of extinction. Marcus wouldn't be an environmentalist if protecting the microbes of a random desert world - she would be a religious nutcase.
All kidding aside, do we know that the criteria for where and when Genesis could be used came from Marcus? As the project leader, she clearly had say in how the device was used, but isn't it possible the "no life at all" dictum was handed down to her by the Federation Council?
It's easy to read too much into her statements to Terrell about not being able to use a site where even the most rudimentary microbes lives, but it does seem that her reiterations of the project guidelines were a reflection not of her own ideas about how her project would be used but of how the Federation believed it should
be used. That Genesis later became a hotbed for controversy because of the circumstances under which it was created only made the situation worse.
The Federation Council stood to be embarrassed by a project they sanctioned because they lost control of it, a loss of control resulting in multiple deaths, the loss of two ships, and endless posturing by the Klingons. It's not clear if that embarrassment trickled down to Marcus. She's never seen again after TWOK, but given the scrutiny surrounding the Enterprise
crew merely because they were there, it's hard to believe she came away unscathed.