How would it be helpful to waste a huge amount of space for the vast majority of text, impose additional limitations on the number of combining characters, just for the benefit of fixed-width encoding of graphemes, which even in itself is not all that helpful since for most applications, you are either going to need to scan through the text linearly, or you could use byte offsets in the variable width encoding just as well as you could use grapheme offsets in a fixed width encoding?
What do you mean by "getting the number of characters actually displayed on the screen"? Do you mean number of glyphs? Number of grapheme clusters? The number of glyphs is dependent on your text rendering system and fonts; even with plain ASCII, you may have multiple codepoints rendered as a single glyph, such as in ligatures like "ffi" in certain fonts. The number of grapheme clusters can be computed by the given algorithm along with some tables of character properties; it's not particularly simple, but trying to represent languages that were originally hand-written on a computer is not simple: http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
In addition, since the existing Unicode standard does not impose a bound, and implementations support stacking of large numbers of diacritics, people have used combinations of extremely large numbers of diacritics as a stylistic device, for example: http://stackoverflow.com/a/1732454/69755. Since examples like this exist in the wild, they need to be supported properly, and so the fundamental string types and operations in string handling libraries should be able to handle it.