I don't believe the Bible would go to such great detail as to delineate the exact number from each tribe of Israel if the intention was not meant to be literal Israel. I wonder why the Watchtower would practice such inconsistent interpretation within ONE verse. How can the number 144,000 be interpreted literally and then in the same sentence a switch is made with regard to Israel...which is then interpreted spiritually? That is not the way to read the scriptures from my perspective.
Scripture and its interpretation is largely a matter of opinion. One 'scholar' differs from another in how they understand the application of the verses. For us, the "Israel of God" that Paul describes in Galatians 6:16, is not literal Israel as other scriptures indicate quite clearly. Scripture interprets scripture for us.
Because that literal nation, with a long history of rebellion and punishment, failed to live up to their willing acceptance of God's law and were clearly complicit in the death of their Messiah, there was a clear statement from Jesus that because of their serial refusal to be corrected, once God had fulfilled his obligation to them by producing their Messiah, he "abandoned" them as his people and chose a new nation under the leadership of His son. (Matthew 23:37-39) A new 'nation', under a new covenant, which Israel rejected and still do.
As we understand Revelation, the delineation of the "tribes" is simply an indication of the order for which God is renowned. The 144,000 are all bound for heaven with positions in a governmental arrangement that would reflect the order that ruling the whole world would require. The order seen in literal Israel shows how important order is to God.
The reason why we believe it is a literal number is because it is contrasted with an infinite number in Revelation 7, where 144,000 are seen in heaven with the Lamb, but an unnumbered "great crowd" are seen 'coming out of the great tribulation, which occurs on earth. (Revelation 7:13-14; Matthew 24:21)
The way all we see scripture, is largely a matter of how we want to see it, to fit in with our chosen religious worldview. We are free to see it as we wish....that is how God determines where we stand on the issue of his worship in this time of the end, and what is in our heart.
That is how we see it.