One must be careful when making assumptions, and so many religions have attempted to define the 144,000 mentioned in scripture.
Where I feel the Jehovah's Witnesses have missed the proverbial boat is in stipulating that there are two classes of resurrected beings: a great crowd and a smaller number of "other sheep" who will be heavenly beings of spirit.
Every man (and woman) have the opportunity of receiving the same resurrection as Christ's, and there's no limit to the numbers. The 144,000 are special servants of God who will help gather the elect of God before the great and dreadful day of the Lord. One also must be careful in assuming that because there is a "spiritual Israel" that it replaces the house of Israel. It's not that God substituted the former for the latter, but that the latter was "adopted" or grafted into the former. "For God hath not cast off his people." (Romans 11:1-2)
If the 144,000 are to be from "spiritual Israel," then why does the apostle take the time to individually list the tribes to be represented? As you stated, those tribes haven't gone anywhere; but God knows who they are and where they are. And if they are priests, even high priests, then they certainly won't take this honor unto themselves, but will be called of God, as was Aaron. And they will be ordained by those with authority. There is nothing to suggest that their resurrections will be any different than yours, mine or anyone else's. They will have special missions, and all will be men (not women); they will go throughout the earth with a special mission. The "great crowd" the apostle sees already are in heaven, "standing before the throne" of God. During the Millennium, the earth will be restored to its paradisiacal state. At the end of the thousand years, things won't just continue on, but the earth will be completely cleansed and glorified, and become like the sun.
In short, people who "inherit the earth" and are made coheirs with Christ, inheriting all that the Father has will be, in a real sense, gods, having authority (kings) and power (priests) to God and His Father.