Omo, I was raised bilingual, so if English entered my life at a later stage, which is likely, it's still part of my "heritage" as much as Dutch is. Besides, my first words were in Arabic, a language don't master.
I only really consider Dutch my first language because I've got a Dutch passport and Dutch parents. I've spent most of my childhood outside the Netherlands. I just don't have an emotional connection with the Netherlands or its literature (which I find poor and monotonous compared to literature from other European countries, but that's not really relevant).
I'm certain it also says something about my lack of talent as a writer, but that's no reason for me to stop doing something I enjoy, is it?
When comparing languages, the differences in vocabulary are a given. They undoubtedly affect the way the languages is written, but you shouldn't overlook grammar either. Kafka, for instance, makes a extensive use of a grammatical structure that places the main verb at the end of the sentence, something which simply can't be replicated in English. The atmosphere that's created with this structure can of course be created through other methods too, but I feel that it's quite fundamentally different (by the way, this is also why I'd recommend reading Kafka in the original German. I found the English translation disappointing and the Dutch translation only slightly less so). From the top of my head, I wouldn't know any other examples, but I'm sure there's plenty.
Ultimately, what I think it comes down to is that I don't think a good author in a certain language, will not necessarily be a good author in another, even if he/she masters that language. Dutch author Gerard Reve, who's held in high regard in Dutch literature, also wrote works in English, which are left unread most of the time, for good reason.
In my experience, it's also easier to write a story in the language of the location where the story is taking place. Obviously, I can't back this up with anything, as it stems purely from my own experiences with writing, but I believe it's true. As I've never written something situated in the Netherlands, I've never felt the urge to write it in Dutch.
Concerning the trend of English literature pushing aside Dutch literature in Dutch bookshops, I'm afraid it's true. I suppose the level of English education is at least partly to blame (according to EU statistics, 87% of people living in the Netherlands could read and write English as a second language), but also the fact the all bar the best and most popular foreign authors are translated to Dutch, while often readily available in English, doesn't do this any good.
It may also be worth nothing that in the Netherlands, you're required to complete a reading list in Dutch, English and a modern foreign language in order to obtain your High School diploma. Ask around on any Dutch High School, and you'll find nearly everyone enjoys, or at least doesn't mind, reading literature in a foreign language, but struggles with Dutch. Complaints I often hear are that it's boring, fatalistic and filled with sex (this is teenagers, mind you! Complaining about the height of the level of sex, just imagine).
Also, I'm a he.