I agree but (1) they aren't the only martyred souls John sees, and (2) the martyred souls of Rev 6 didn't remain under the altar waiting for Christ to redeem them from the graves. After His cross and resurrection, we see them again in Rev 7 as 144,000 from all the tribes of the children of Israel being sealed with the life-giving spirit, and then we see the same 144,000 again in Rev 14 appearing with the Lamb on Mt Zion of heaven (Heb 12:22), as firstfruits unto God having been redeemed from the earth. If this isn't evidence of living spirit souls in heaven after death, I don't know what would be.