This all would have been avoided if they'd started the movies at a different point.
You want to get rid of the old cast and focus on new stories? That's a good idea. Set the plot somewhere outside their sphere of influence, or maybe have one old 'PC' as a side character to help guide the new generation, like they did with Han Solo in TFA. But it makes for unsatisfying storytelling to leave off-screen a bunch of tragedy that dramatically changes the personality of our main characters.
If you want to show Luke failing as a teacher, his order being destroyed, and him withdrawing from his family and friends to focus on his shame, okay, have that be one plot thread of Ep 7, which could show Ben becoming Kylo, while keeping roughly the same character beats for Rey and Finn. You can then have Ep 8 involve Han trying to save Kylo and that failing, and have the Republic destroyed by the First Order, with the focus on the new characters becoming heroes of the rebellion. Then maybe Ep 9 is the rebellion failing, but still being a spark of hope to the next generation.
But it's . . . it's bad storytelling to skip over perhaps the second most dramatic thing to ever happen to Luke Skywalker, especially if you later on want that moment to matter to the characters. As much as I complain about the plotting of this new trilogy, Kylo's course has been enthralling, and I think actually seeing his first turn to the Dark Side would have been great cinema.
Think of it like RPGs. If you roleplay through something, and the players fail, that's sad but acceptable. But if the GM says, "While you were sleeping all your gear was stolen and the bad guys rushed ahead and finished their evil plan," that's bad storytelling.
