Whether March 25 predates the absorption of pagan holidays doesn't counter the point that I made: That many parts of the liturgical calendar are a result of Christianizing what were pagan celebrations. Why not continue to do the same, and make so that our feasting and fasting days are congruent with the how our cultures holidays have evolved. We can re-Christianize it again.
I would describe what happened somewhat differently, namely that the popularity of certain Christian feasts were perhaps greater because they coincidentally fell close to certain pagan festivals.
This is obvious from the fact that in different parts of the Christian world, different feasts take on different levels of popularity, depending on what pre-existing local festivals were there. Old habits die hard, and that isn't necessarily a bad thing.
I once remarked to our priest that I found it interesting that at our parish, our Wednesday evening vespers service is pretty well attended, often better so than vespers services for higher ranking feasts that occur on other days of the week. He pointed out that in America, Wednesday is traditionally "church night," and that the many converts to Orthodoxy are already used to going to church on Wednesday night.
Likewise, we always have an excellent turnout for liturgies on Thanksgiving Day (regardless of what feast falls on that day), and on New Year's day (for the Feast of the Circumcision/Feast of St. Basil the Great.) This is because people are off work on those days.
These sorts of things, of course, happen naturally.
The question, of course, is whether Christianity transforms culture or whether it follows it and is blown with whatever cultural wind exists.
Modern Christianity in America doesn't seem to transform culture, it seems to follow it. It doesn't displace secular society, it comes up with poor "Christianized" imitations and substitutes. Just look at the drivel that passes for Christian music, especially these days.
At the core of the successful transformation of the pagan world into Christendom was an opportunistic Christianization of paganism, but rather the willingness to have a radical discontinuity with the surrounding society. The earliest feasts were those held on the anniversaries of the martyrdoms of Christians slaughtered by the surrounding culture.
Orthodoxy is not going to change its cycle of feasts and fasts. But if Protestant churches are able to find ways to transform, rather than imitate, the surrounding secular culture, more power to them. I just haven't seen that American Christianity has the strength or aptitude for it.