I think people focus more on GAC omis over TW omis since GAC provides better rewards. Plus having 3 omi abilities on some characters vs only 1 omi ability for tw characters it makes more sense to go after rewards in GAC. Thus the cost decrease. But it was also only directed at characters with the 3v3 and 5v5, not all GAC omi abilities, so it was a small enough pool that was affected.
Maybe it would make more sense to balance the amount of omi abilities on tw omi characters by giving them 2-3 abilities that can have omis as well, or at least amplify those abilities a bit more to balance like the gac omis do for gac to make them more worthwhile.
I like TW over GAC, so I would be on board with a form of this idea, but reducing TW omi costs might just be awful as then suddenly everyone would start getting omicrons back and it would just make GAC and TW that much more difficult with a sudden increase of everyone enhancing more characters. Datacrons already made both more difficult.