I wholeheartadly believe that new generations can't be forever better in everything than the previous ones. There will come a time of stagnation or even decline.

So there is absolutely nothing wrong in decline. It's mathematically necessary. (Well, stagnation, or slow increase is also possible.)

I also don't think that the only function of the education system is to score higher and higher on tests, it has so many other functions: keep kids happy, turn kids into happy adults, lower the tensions is society, create a better world for everyone, etc.

There wouldn't be much point of scoring better in tests if it resulted in unhappy kids, unhappy adults, broken society, broken world, now would it?

I'm inclined to believe that the decline of our education system is intentional. Certain people don't want the masses to be capable of critical thinking.

Decline since when? A few hundred years ago most people were illiterate farmers.