Fair enough but there is no common fallback font that is used by all systems (win, osx, linux, android ...) as a default.

Take this page for example: http://emojipedia.org/faq/ Only a fraction of the emojis is displayed on my laptop. (maybe 20%)

As a data point, my laptop shows every emoji through Unicode 8.0 just fine.

> there is no common fallback font that is used by all systems (win, osx, linux, android ...) as a default.

Again, meaningless. Why should there have to be a common fallback font?

to make sure that even the most idiosyncratic choice of characters can be displayed everywhere according to the intention of the person who picked them.

Unicode supplies sample glyphs for all characters. That's plenty.