No, you're right--it would certainly be possible to substitute any suitable higher-order group there (if you were synthesizing some compound).

But the beauty of the convention used in that type of diagram is that, unless otherwise specified (by, for example, attaching another line segment to the dangling endpoint, or by writing an "N" for a Nitrogen or so forth), it is implied that only a carbon is positioned at the end of the line segment (as well as enough hydrogens to complete all three remaining bonding sites, again unless otherwise specified).

See? I think it's pretty clever, really, once you grok it (took me a while at first, too, but then I always was pretty slow).