The GA matchmaking currently only uses your top X characters (I believe the range is 40-80 depending on division), so your rage is a bit misdirected. Whether they are 1 star G1 or 7 star G8, it's functionally irrelevant for matching purposes.
Your complaint about the mismatch in G13 characters on the other hand is more valid. Currently the GP delta between G12+5 and G13 is only about 1%, even though the performance of G13 characters is most certainly >1% better than G12+5. Increasing the value of all G12 pieces, particularly the finisher, would greatly improve the likelihood of competitive matches, and I believe it's something CG has already acknowledged in a Q&A.
Also, there are like 20 threads on this topic.