Well, they were doing a thing like it since then at least.

But not on weekend mornings. Rider and Sentai used to air on weekday nights. The question was whether the schedule could change. Precedent says yes, though I wouldn't hold my breath.

We could consider Goseiger a science-fantasy sentai ?

No they weren't. Sentai and Rider never aired back to back until Kuuga/Timeranger. Metal Heroes aired with Sentai from around 93 to the end of its run, but before that there was never any sort of superhero show block.

Kamen Rider and Super Sentai didn't exist in the 1960's.

Then why the hell does Wikipedia bring up the 60's on that page?

You should not trust the Wikipedia since it can be edited by anyone unless they have a legit source to back it up.

Because there's always been a kids' TV block running during that time slot. Super Hero Time took over that slot in 2003.

Though I would like to point out Japan's wikipedia pages regarding stuff like tokusatsu tend to be and seem more reliable compared to the English pages.

Yeah, there's no mention of a superhero block before 1997 on the Japanese Wiki. Superhero shows have aired in that timeslot in the past, yeah, but that doesn't mean it always some sort of block.