I think that someone of Scottish heritage who creates a tartan to reflect their more recent lineage and home is most certainly not "turning their back" on their ancient lineage, but rather bringing it forward, alive and kicking, into current relevance for themselves and their descendants.
I absolutely agree! Maybe I should have worded it to mean that inventing new tartans out of a sense of guilt for 'hijacking' Scottish tradition would be turning one's back on his ancestry. I think an evolution of tartans to honor new family ties is great. But it shouldn't be done from any sense of pressure from native Scots. There should be room for both honoring our ancestry as well as forging a new identity.