No one needed to explore before 3.3. That is a player choice.
That does not find everything that there is in a system. Sure it finds every planet and asteroid belt but that is no different from the old ADS. Or do you think having a first discovered tag on it means you have explored the planet in the old or the new system?
So how do you explore planetary surfaces by not exploring the planet surface and flying there with just the FSS?
How do you see strange steller phenomonen if you don't fly to lagrane clouds and other places with just the FSS?
How do see strange biological things on planets if you don't fly to them and just use the FSS?
How do you explore gas giant rings with just the FSS? How do you look at eclipse (you can't do that sitting next to a star)? How do you see strange planetary alignments. Do you see all those with just the FSS.
The FSS discovers these, like the old system, but it doesn't explore them for you. Now go and explore those places by flying to them and discovering what is there.
What you are saying is like an explorer discovering a mouth of a river and saying they have explored the whole river. They haven't. They have just started with the exploration process and it's same with the FSS, it hasn't ended it, it's only just begun.