We have a darkness aligned DW, whose stats scale with the number of darkness meridians we have active. Your argument is invalid.
The number of darkness aligned Meridians we have will only increase, even if we only pick up Arts that are direct successors to our current suite, solely by virtue of more powerful Arts requiring more Meridians, in conjunction with the ability to scrunch Meridians down not reducing the 'total number' for such purposes.
And it's not as though I'm not hoping for a successor to TRF that is somehow Wood/Darkness/Moon, or a Darkness/Larceny buff-stealing Arm/Lung or Arm/Heart Art, or other, more esoteric things (Moon/Sun/Darkness 'Eclipse' Art, anybody? Not sure what it would actually
do at the moment, really should go to sleep).
Your argument is lacking in any other points. And though our Domain Weapon is important, it isn't the center point of our build. Choosing new Arts based upon our Domain Weapon, something that we can change and replace if we so desire, is foolish - it should be the other way around; we should choose a Domain Weapon based upon the Arts we want.
In addition, it will eventually be changed to something else. Probably in Fifth Realm, or thereabouts, when we've finally moved beyond our current one's potential - a long time from now, granted - but we don't have to wait that long if we don't want to, and a Domain Weapon that better fit our Meridians would be remarkably useful. If we can ever figure that problem out, at any rate.