Back in 2001 the old Sci-Fi game Traveller was ported into the d20 system. The classes were based on the non-d20 version (army, navy, merchant, professional and so forth) but they went up to level 20.
Starfinder should also have rules to take the classes it uses all the way up to 20th level, so that any campaign which makes it that far has rules for it (and so that anyone who house rules has something to base it off). Adventure paths? Those should go up to whatever point the author(s) consider dramatically appropriate, whether it's 20, 15 or even 5.