I'll give two examples.
Netflix focused on productivity. They even popularized the phrase "undifferentiated heavy lifting". They were able to build a metric system, in less than a year, that supported full OLAP query with practically arbitrary number of dimensions. The system ingested > 20 billion data points per minute, ran on more than a thousand machines, and did not set quota to any team or service. The entire system was built and operated by merely three engineers. This was simply not possible without AWS. Similarly, Netflix's data pipeline, Elasticsearch cluster, and Druid clusters were developed and operated by two engineers. They were oncall 24x7, yet they rarely got waken up in the middle of a night. You think this was possible if Netflix tried to build its own infrastructure?
The counter example is Uber. Uber was ambitious and wanted to build everything. The results? More than 200 alerts got fired within 2 hours -- the poor oncall engineer couldn't do anything but to keep ack'ing the alerts. Wanted to use Cassandra? 10-page forms that were more tedious than applying for the Ivy schools, weeks of waits, lots of angry emails, countless meetings, and still no Cassandra instances. Wanted to add 50 machines to your ES cluster? PPTs, meetings, escalation all the way to CTO, and weeks of planning. Want to try out Apache Kudu? No way, because our container-based EC2-like system did not support persistent volume, let alone something similar to EBS or EFS. Wanted to use container for your ES cluster? Same answer. Wanted to create your bespoke cluster topology? Same answer because apparently custom network configuration was not supported. After waiting for more than three years? Same answer. If this was not expensive, I don't know what was.
So, you asked if you should save a few thousand bucks a month for your fast-growing company?