To give more details on the story about my students.. At 14-15 years old, they were unable to perform subtraction correctly with single digit numbers.
All of which because they uniquely used tablets and qcm for all their work. This was a general issue with their capacity of concentration, memorization, and overall learning.
They were able to understand a simple concept, for example symmetries of objects, after a lot of work, but forget about it the following day. I helped them through some years. They are doing better now.. But they didn't catch up what they should be able to do as functioning adults.
If you don't teach to a child to study, he will never do it, studying is not fun per se, if it's not motivated, the random probability that one does it alone is extremely small, and even then. You fail him for not providing him a good education.
Now I hear you, you are talking about preschool and, clearly I think that prematurely forcing them to study has no purpose, they should start studying when they are ready to it, but we do not have a way to know when it is.
I'm on the camp of making study as fun as possible, not by creating random games around the work, but instead by providing lot of context and unexpected examples that motivate the creation of the theory. In this way, I think education can start pretty soon.
I like to have my student study because they are intrigued instead of having them study because they 'need to'. Which is counterproductive no matter the age.