You are always paying, either in time or money. Which you pick is based on your assessment of where resources of time and money are best spent forward looking. Some people do their own laundry. Some pay a laundromat to do their laundry. Why do some pay? Because those that pay have decided to trade money to buy some time back for another (higher value to them) activity. There is no right or wrong answer, only tradeoffs based on current ground truth and predictions.

Paying Datadog (or whomever you elect to manage observability) means you're not paying someone to or spending time focusing on said observability. That is time you are presumably spending on development velocity to accelerate revenue growth. Balance growth and optimization accordingly.