Does this come from actual experience trying to communicate in chinese or japanese? At least for chinese (but I'm pretty sure it applies to japanese too), what you're saying doesn't ring as relevant (i.e. it is true that some characters don't have a graphic representation in many fonts, but it doesn't cause problems for communication / creation of new words)

Plenty of fonts can represent a vast amount of characters (search around, but even free fonts can have >50k), much more than even highly literate people could ever use (a very educated person might know 8k characters, but would use much less actively).

Every new release of Unicode releases new characters that are slowly incorporated into fonts...but for many years these are on the truly long tail of characters, nothing that any common person would use. There might be a few exceptions with new characters (the new japanese emperor characters comes to mind), but the thing to bear in mind is that new words don't necessarily involve new characters. A big chunk of nouns in chinese are composed of multiple characters, so making a new combination can generate a new word (电视 is television...electric + vision). This is probably even easier in japanese were they can use katakana to spell an English word and create a new japanese word.