Tenured employees help maintain culture, but they don't guarantee that it stays static.
Culture developed by the employees who spent their first 5 years growing with the company is not necessarily the same culture developed by those same employees who have stayed in the same role for the last 5 years with a vanishing potential for growth.
On the other hand if the culture you want to maintain is "constantly be trying out new technologies, new ways of working and fresh ideas, at the cutting edge nothing is set in stone" you might benefit from new employees who know firsthand how other companies solved whatever problem you're facing.
And the conditions for most big tech companies have changed radically over the last few years. So the subtle devaluation or reinterpretation of culture is kind of inevitable when forever-growth turns into a zero-sum game. As you say, it might even be the veterans who are doing it.