You can use whatever curve, shape and symbol you like, but the unicode U+0079 should essentially represent the letter 'y' (rather than 'umbrella'): that's precisely the point of it.
If I don't get your symbol, I should be able to revert to a font that I do get.
Symbols are random things that mean nothing until someone says it. If you and I agree that U+0079 is 'y', then so be it. But lets use the other namespaces of the index, too .. and in that regard .ttf has much to offer the aspiring user of the format. Problem is, a shallow look at it, of course, renders the conclusion that it is an investment; but if I know for sure my glyphs render the same way for grandma, no matter if she's using it on some grungy screen, somewhere, then I'm okay with letting the heart-beat monitor screen be composed mostly of cache-able sub-functions derived from somewhere_safe.ttf.
On the other hand, using SVG to pre-load values, then modifying the decomposed stack, somewhere appropriate, in order to attain sustainable realtime SVG rendering performance .. seems as if its a matter of principle. Could it be the jury is out until someone says screw it, and bases the entire OS of their new device on SVG, alone, and not much else?
I think the recent xkcd about symbols is very relevant here, interestingly. As more and more folks were having fun with icons, whether fonts or otherwise, to represent things, it is hilarious how many more slight variations of a symbol I had to learn. Only to usually have sites revert back to using the words at some point.
That is, arguing that symbols are random seems silly. The whole point is that there is a large base of agreed upon symbols affixed with meanings already. They can even be combined to form different already agreed upon symbols.
If I don't get your symbol, I should be able to revert to a font that I do get.