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 gunas 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 guna 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-guna can overwhelm tamas and sattva, and tamo-guna 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-guna should be on the ascendance. That alone will enrich the individual and society alike.