Yes, it would indeed be a very bad thing.

A European Super League in the format that has been suggested would greatly devalue the Premier League, which would in turn greatly devalue the Championship and the rest of the English pyramid. It would kill off the League Cup (maybe not such a bad thing) and the FA Cup (a very bad thing). It would greatly devalue the Spanish and Italian domestic leagues and it would pretty much kill off European club football as we know it.

It would also be very, very boring.

It is an idea that has no upsides whatsoever.