The de- celtisization(I know, it's not a word)began when David became king.
Raised in the Norman court, he was taught that Scots were uncultured savages. he brought with him to Scotland Norman knights who were mostly landless and were hoping for a shot at a position or an estate. Many came later. David began consolidating his position and instituting the Norman system of administration, which was also a way of weakening the position of clan chiefs. Court-wise Scots looked to marry into the families of the king's favorites, most of whom were Norman. The newly created positions of authority went to them, creating a power base for him and a wedge to begin moving all power to the throne. Clan chiefs owed their position to the clan, which benefited from able leadership that knew the lands were those of the clan; all stood to gain by standing together. The Norman view was that all land and all that was on it, including the people, belonged to the throne. The heraldry and sumptuary laws grew out of this effort to control first, govern when convenient, and crush or remove resistance. Clan chiefs and their tacksmen began to believe they were better than the clanspeople,
and that their authority was a gift from the king. The clans suffered increasingly, coming to a head in the clearances. These "common classes"
as some refer to them, could not afford so-called Highland Dress, and as you say did not commonly wear it. The gentry did have it, and did wear it, and that's why Scott was able to put out a call for people to show up wearing
it for his reception for the king. And on short notice. They didn't have to pay 1300 pounds for it, they already owned it and wore it. It was in use in the Highlands, in common use, just not by the common man. Perhaps in many cases not EVERY day wear, but seen. No question that it owes much of it's longevity to that visit, and to its' use by the military. Those uses were
based on its' prior existence, but without those uses might have disappeared.
Only conjecture is possible. What is not conjecture is that tartan has been in
use for a very long while, and kilt-like garments as well. Their evolution into what most think of as THCD did come later and does owe much to the visit.
My folks were already gone by then. This is why I do not wear the "rose-coloured spectacles" decried in prior threads. I don't have a dog in this hunt.
I only have interest in how it all went down, I don't gain or lose by it's being this way or that. History is a fabulous (deliberate usage) spectacle, and I take joy in it, warts and all.

BTW, take a look sometime at the seals of 12th century seals of Scottish
kings, at how they are garbed.