Early Scotland wasn't 'Scotland' at all though. It was as you say gaelic, firmly in the north. The majority of Scotland's power and most of its population are in the lowlands.
Yeah in times past this wasn't so great a issue due to less people all around and other factors but if/when Scotland developes into a prosperous nation the Northumbrian cultured lowlands will win out.
At Northumbria's height it ruled the lowlands.
That's how so much of 'Scottish culture' is mostly just a variation of northern English.
Probably more accurate to say that Scottish culture is a fusion of different origins, including northern English. Linguistically, it is closer to English, but it would be a mistake to think of the Kingdom of Scots as simply a northern version of Northumbria, and it would also be a mistake to presume that Gaelic culture could not move south, given the right circumstances. After all, it only took a century or so for Anglian culture to take hold in current day Lowland Scotland - it would only take a century or so for it to change back, less in fact, as there were many areas where Gaelic or Old Welsh culture survived for a long time after the Northumbrian takeover.