Adho gacchanti tāmasāḥ. Therefore the human civilization means to make progress towards the quality of goodness. That will help him for making farther progress. Ūrdhvaṁ gacchanti sattva-sthāḥ (BG 14.18). Ūrdhvam means higher planetary system. Here it is said that api trailokya-rājyasya hetoḥ. Trailokya. Oṁ bhūr bhuvaḥ svaḥ. This is bhūr-loka. Above this, bhuvar-loka, and above that, svar-loka, heavenly planets. Oṁ bhūr bhuvaḥ svaḥ tat savitur vareṇyam. This is Gāyatrī-mantra.
So Arjuna is speaking of trailokya-rājyasya. He is ascertaining that "Even if I get the kingdom of the three worlds, what to speak of only this earthly planet, what is insignificant, even if I get the kingdom of trailokya, svarga, martya, pātāla, like that, still, I am not prepared to fight with my kinsmen, what to speak of this earthly planet." He decided like that. This is called family attachment.