This is the final paragraph in the discussion section of the paper by Skulachev, regarding the use of a Na+/K+ concentration gradient across a membrane to store potential energy, convertible to a Na+ or H+ gradient as needed, and why elevated K+ does not have to be a primordial feature of proto-cells:
"One might think that Na+ ions are incompatible with life and this is the reason why K+ is substituted for Na+ in the cell interior. Apparently, it is not the case as, e.g., in halophilic bacteria [Na+]int can reach 2 M . The very fact that some enzyme systems work better in the presence of K+ than of Na+, may be considered as a secondary adaptation of enzymes to the K+-rich and Na+-poor conditions in the cytosol . Besides, it would have been dangerous to couple any work performance with Na+ influx to the cytoplasm if Na+ were a cell poison".
That makes perfect sense to me.