O Bhārata, (sometimes) sattva prevails dominating over rajas and tamas; (sometimes) rajas rules over sattva and tamas; (and again) tamas overwhelms sattva and rajas.
In every one, all the three guṇas are there. But their proportions will be different, and they will vary with time. Our intention is to enhance sattva, giving it the leading position, and to that extent reduce rajas and tamas. But presence and participation of all the three are indispensable.
Given the right persuasion and effort, each guṇa has the potential to enhance its measure, overwhelming the other two. Thus sattva can be predominant, making rajas and tamas subservient.
The same way, rajo-guṇa can overwhelm tamas and sattva, and tamo-guṇa can dominate rajas and sattva. In fact, Nature preserves four distinct groups of tendencies in this manner. Krishna has already defined (4.13) these four categories, which enable the society to be cohesive and efficient, each complementing the rest.
For a spiritual seeker, sattva-guṇa should be on the ascendance. That alone will enrich the individual and society alike.