There's positives to anything that happens.
If they go bust we can all have a good chuckle as their fans go glory hunting elsewhere and we become undesputedly the only club in Manchester.
If they become a poor side playing in division 1 or worse they'll lose their stadium, fans, revenue etc. We can turn the old joke back upon them. And it'd be interesting to see if they could claw their way back up like we have (my opinion - no. Their support isnt good enough like ours has been).
If they become a poor Premiership team - MY PERSONAL FAVOURITE! here guys - we can have two derbies a season - minimum, and hopefully trounce them. They'd hopefully be in the relegation dog scrap every year.
If they remain a thriving force in the top 4; GREATER Manchester would become possibly the strongest region in football and also the rivalry would become one fans all over the world would want to watch. Part of me likes the idea of "Manchester City beat Manchester United to the title" every year - a Greater Manchester 1 and 2. To be fair to a lot of my less arrogant stuck up rag mates they'd like to see City in the top 4 (admittedly winning as little as possible though).
It's win/win in my opinon as long as we can push on now and become the footballing superpower we want to be with cups and champsleague football.