I think the correct word is a "changing" company. In a static environment, without competing products like IG, FB etc, Twitter can afford to stay the way it is and probably do incremental improvements.

However, in a competitive environment, and with proof that the market is expanding (Newer generation of kids are using social media), if it fails to capture the market, the company will die out.

So, there is this need to focus on user number growth.

I am sure that if the company was only adding older people, the stock would be punished inspite of growing numbers.