I'd argue it's a combination of both. Facebook is a good example of this. I still remember when Facebook was still considered a cool place to be, and it was more of where highschool and colleged age people gathered. Once everyone and their grandma got onboard, that's around the time it started becoming insufferable.

A lot of people who were here before the reddit migration definitely noticed a change in the culture here (and complained about it in "unpopular" opinions) Having a large influx of users is absolutely going to change the culture of this place, even if the advertisers and celebs never come.