Plasma isn't like antibiotics. There's no enzyme that can degrade it, and no transporter that can pump it out of the cell. Its damage is untargeted, so the only way to resist it is to become generally tougher, which has high metabolic cost. The microbes that don't evolve this will out-compete them. Worrying about resistance to plasma (or similarly indiscriminate antimicrobials, like bleach or ionizing radiation) is like worrying about animals evolving armor plating to resist bullets.