I couldn't agree more. Far too much aesthetic emphasis is given to elevating the surface style of music - or indeed any other narrative artform; cinema, literature, even painting - above other deeper elements that are often more fundamental to making music music. Specifically, what a piece sounds like has only a limited role in defining its worldview and aesthetics, and often has the irritating side-effect of pigeon-holing it under some vague term as 'classical', 'romantic' and so forth. I've said this before but there are certain deep aspects of music that are common to almost all works even beyond the western classical tradition. Obviously we are writing in precisely this tradition and so can widen our aesthetic Venn diagram to include only things fundamental to it alone, but the point remains. What this means is that whilst they may sound very different, that squeaky modern piece you heard last week might actually have a lot in common with a Mozart quartet. It's of limited use to seperate the two in terms of compositional tecniques just because one is atonal and composed on a computer program and the other is based on decorating a functional harmonic pattern with a melody (if this is even how we should analyse Mozart). Perhaps finer study might reveal structural similarities, or that certain devices both composers use are intended to have the same effect or the same function, or even that the 'intensity curve', the narrative of the piece, is very similar. Another way to think of it is with a diverse group of dogs. They may all look very different - big, small, hairy, cute, aggressive - but they have a common ancestor and over 99% of their genes will be identical. So it is with music. Fundamentally, the elements that affect the listening experience are the same. What varies is the composer's choices regarding the deployment of these elements, for which there is great scope.
What does this have to do with formal training? Firstly, it should be the responsibility of every composition pedagogue to emphasise this more fundamental aspect of musical creation and to regard surface style as being only of arbitrary interest. It is also as bad to only study the most modern avant-garde music as it is to ignore it completely. I remeber putting my hand up in a composition seminar entirely devoted to contemporary works and asking 'Is there anything we can learn from Haydn?' and getting a funny look and some dismissive answer from the lecturer. (Luckily my next teacher was more broad-minded). Teachers should ensire students see and understand as diverse a range of music as possible and most importantly introduce the idea that the vast majority of it is still relevant to the contemporary composer. Secondly, avoid labelling more than is neccessary as it will comporomise this view and encourage the student only to write in a 'contemporary' idiom (for which read a contemporary surface style). If I write a completely atonal piece using electro-accoustic overtone analysis but arranged the material in prototypical sonata form, is it a 'classical' or 'modernist' work?
To ask the question 'why do all modern composers write is a modern style' is disingenuous: the answer is, they don't. What they do do is write using ideas and techniques borrowed from other musicians according to their own aesthetic choices, the end result of which eventually is labelled 'modern'. That should be your training.