There's no incentive for the Chinese gov to use military force, period. What the protestors are doing is actually pretty beneficial to the Chinese gov. The protest is destroying HK's economy, yes. But HK's economy is actually not vital to China's economy anymore[1]. Instead, a less economic significant HK would be much easier to govern in the long run. China would rather further develop its Shanghai or Shenzhen, than investing into HK.
What's more, the recent trade dispute with US, and the Hong Kong protest, have made significant negative impact on mainland Chinese attitude toward the western countries and idealogies. If there's any internal conflicts and fights for certain types of freedom before, they certainly become muted now. The rise of nationalism is surely making the higher-ups happy.
1. https://www.vox.com/2014/9/28/6857567/hong-kong-used-to-be-1...