Population decline by itself isn't a bad thing. The rate of population decline is what is worrying. 
I live in a country that had just under 4.8 million people 30 years ago. According to last year census, we are just above 3,8 million now. By the end of this century we will be perhaps below 2 million. Whatever your political views, the idea that that sort of population decline wont have negative effects on the country and society is completely delusional.
And we are not an exception. Developed countries around the planet are going to have rapid population decline in coming decades.

Innovation comes from a well educated population. Not from a large population. 

If you live on a planet that has a bunch of old people, instead of young people, that means that a greater proportion of money and resources that would have otherwise been spent on education or innovations now has to go towards medical care for the old. Which means less innovation. 
Also. All other things being equal, would a planet with a million people or 10 billion people have more innovation ? The answer is obviously 10 billion. Larger well educated population will have more innovation than a smaller well educated population. Didn't think that was controversial.