A new notation system will need to be a lot better to justify the change, because there is also a lot of value in compatibility with everything that already exists.
I'm not sure a sufficiently better system exists, because as you say, the traditional notation works. It has its quirks and rough corners, but music is complicated enough that any system would probably have similar imperfections.
Which in durations are represented spacially, which in my opinion has two negative effects.
The first is that spacial recognition takes more effort than symbol recognition, because it's comparative.
The second, and more important being that complex sequences of notes will be very dense on the page, and simple sequences of notes will take up a lot of space on the page, so suddenly there is a tradeoff between having sheet music that doesn't take 10 pages, and having enough space to represent hemi-demi-semiquaver sequences when they inevitably appear somewhere.
Yeah wtf. Like I don't think current notation is perfect by any means (and I am a PLer, I love new representations) but this and every other replacement I've seen blatantly sucks. ....As with so many things, know what your disrupting!
[In this case, I'd like to say go apprentice engraving if you are 100% serious.]
With Hummingbird's inclusion of a trailing line as part of the indicator of note duration you also have to read ahead in the score and then jump back if you want to use the trailing line to identify the duration, which is a lot harder than just identifying by local information in the form of a set of note tails and whether or not a note is filled in. While there are additional parts to the glyphs for half and whole notes, these aren't the dominant part of the symbol.
Another question is whether or not the length of a trailing line is absolute or relative to the bar itself.
On the subject of filled notes, Hummingbird is also conveying a lot of information that for performance of a score is useless. Note letters (A-G) aren't actually important for performance, only the action or position that they map to for each instrument. No musician parses a score and translates each note to a letter and then each letter to an action, instead going directly from note to action.
Essentially, telling you the note letter with a glyph shape on top of the position on the stave is adding noise to the signal.
I'll admit I'm coming at it from a position where I'm perfectly comfortable with traditional notation, so part of the reason that it appears difficult is simply because it's unfamiliar, however the terseness of traditional notation and ability to read in one "parse" without forward- and back-skipping seems to give it the advantage.
Your second point seems pretty valid, and to address your first point:
"There are multiple cues to the same information. Everything has both a symbol and spatial element, for all kinds of thinkers."
Indeed, in addition to the spatial length of the notes, there is also a symbol next to the notes denoting their life. You can see this on the linked page, in the second section. (Next to "Intuitive.")
Their pages claims: "It’s quick and easy enough to write with an unsharpened pencil. You can scrawl it on a napkin in a pinch."
I don't know if it's true or not, as I view Hummingbird to be fixing things that aren't broken (is the difference between a whole note and a half that hard to suss?) and doesn't fix the things that are.
I think the more difficult part writing by hand would be the redundancies -- not only do I have to know which line to put the note on, I have to know which symbol to draw. Which could be tough, as it forces the composer to be consciously aware that e.g. "this note is D#" rather than "this note should be two steps above the previous note in the current key".
That works, and would be an improvement, so long as your rhythms are based on constant integer subdivisions.
It's possible (and fun) to play music where the rhythmic structure changes smoothly and continuously. But it's bloody impossible to notate and very difficult to orally communicate, so music cultures that depend on notation or oral communication have left this territory largely unexplored.
The handful of classical composers that have attempted this (Steve Reich, Brian Current, etc) have either abandoned western notation (Reich) or hacked on their own bespoke glyphs with their own situation-specific explanations (Current).
Electronic musicians can easily explore this space by writing their own software (Autechre, your humble author, etc). When your musicians are mechanical, you can explore all sorts of otherwise impractical permutations of theory.
Most delightful, though, are the non-western cultures that communicate musical ideas entirely without written notation or spoken language, and instead communicate musical ideas through play (Indonesian Gamelan, Australian Aborigines, etc). You get the ineffable human qualities that make music most beautiful, and the freedom to explore structural spaces that are difficult to capture with discrete/unitized/quantized notation and language.
> But it's bloody impossible to notate and very difficult to orally communicate
It's really easy to orally communicate. You can just sing it!
(I know what you mean, is it's difficult to describe using a computer keyboard. Using a pen and paper it is really easy, given that you can just draw notes and time signature changes in the margin.)
By default the next note is half as long as the previous. But sometimes that's inconvenient. For example 9/8 is usually 3-3-2-2-2-2-2-2-2... A bar is devided into 3 dotted quarters and a dotted quarter into 3 8ths, and anything further down is split in half as is normal.
The answer is, because it works, and nobody has managed to propose a better one.