It's fairer to the students. And by "fairer" I mean in the traditional "anyone can grow up to be President if he just pulls himself up by his bootstraps" American definition of "fair"; unlimited social mobility if you happen to be bright, ambitious, and totally sociopathic.
Almost every culture has its myths of the poor-but-bright student who can't afford to attend school but gets a job sweeping floors at night and reading the chalkboards after the "real" students leave and by the third reel is leaving all the rich kids in the dust. Most European school systems, perhaps especially in the UK, were very strongly class-based, where "his name has been down for this school since before he was born," because every Riddle of Riddleham has always gone to King's College, Riddleham. And, of course, KCR also had much better facilities than the local chapel school precisely because the Dukes of Riddleshire had been funding it for six hundred years, while the chapel had to make do with what they could.
Putting all students into the same system and even the same school avoids this problem; everyone has the same opportunity and the bright/motivated/sociopathic ones will be able to take appropriate advantage of that opportunity.
I can only speak for Holland here; but your concern about strong class distinctions are quite absent. There's also no tradition of alumni or parents significantly funding schools or universities, so they are dependent on the equal financing they get from the government. The only elitistic hold-over in the Dutch system are the separate
gymnasium schools, and sure there are families who have a tradition of going there, but they were (already) a small minority when I went there - and I sure enjoyed attending a school with only 300 students. Most of the Dutch secondary schools offer 2 or all 3 tracks of secondary education.
And what happens if someone reads Cosmos their first year of VMBO and decides that they really want to become a cosmologist?
You may note there are several arrows in the
diagram. There are several possible routes for your student. She can ask the principal to get promoted to second year of HAVO - rule of thumb is that an average grade of 7 is needed (6 = pass) - or even to second year of VWO with an average grade of 8. That's particularly easy when the school offers all three tracks. Such schools will often also make no or fewer distinction in classes between the different tracks in the first year.
The second possibility is that she graduates from VMBO (theoretical variant), and then goes to 4th year HAVO and graduates; then goes to 5th year VWO and graduates; and then studies astronomy at university. One of my friends from university followed this route.
The third possibility is that she follows 4 year VMBO; then 4 year MBO; then 4 year HBO; and then goes to university, where she gets fast-tracked, or that after 1 year HBO, she enters university to do a full study.
Finally, when your student only discovers Sagan after she's done VMBO and has already entered the job market, she can enlist in evening-VWO, and then do Open University or a part-time study at a university.
So, there are plenty of routes to "correct" a previous mistake.
Which is why American schools try to give everyone the knowledge they could need/want later, even if they just want to be janitors and mechanics.
Well, we could obviously discuss what body of knowledge that is, and whether the emphasis should be more on the "need" or "want" part. The Dutch system is obviously more suited for the "need" part.