True but I think that is also problematic.
The more symbols there are the less likely it is that every font covers all the symbols that someone might find important.
For instance, I often get e-mails with characters that can't be displayed because my standard fonts (on linux) don't have them. The "missing unicode symbol" icon is the new "picture not found" icon[0] In fact, it is even worse as there is no alt attribute that delineates what I am supposed to see.
> the less likely it is that every font covers all the symbols that someone might find important
That is a meaningless requirement. The symbols I use on a daily basis already don't exist in a single font. Operating systems handle font fallback just fine.
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.
> The more symbols there are the less likely it is that every font covers all the symbols that someone might find important.
Why would every font need to cover them in the first place? Just one is enough with symbolic characters, sufficient to display them. Text renderers can deal with that.
No font coverage is also fine, at least the data is preserved.
[0] Remember this one? https://pbs.twimg.com/profile_images/425274582581264384/X3QX...