Valve has shared a blog post detailing how new releases have been performing over the last decade.

According to Valve:

• Over the years, there's been a continuous increase in the number of games achieving success on Steam.
• Earnings prospects for most - but not all - games improved in 2019.

The metric for success is whether or not a game earned $10,000 USD within the first two weeks of release. Most games earning $10,000 in the first two weeks earned between $20,000 and $60,000 over the following 12 months. Valve shows that in 2019, the number of games meeting this metric increased 18% over 2018. This graph shows the growth in number of games earning at least $10,000 in the first two weeks since 2005:

They also noted the jump from 2013 to 2014, saying it reflects the increase in number of games accepted to Steam beginning in August 2013.

In addition to the blog post with more information, you can view the full methodology and more detailed graphs in the research appendix.