Cost is usually not a huge problem beyond seed stage. Series A-B the biggest problem is growing the customer base so the fixed infra costs become a rounding error. We've built the product and we're usually focused on customer enablement and technical wins - proving that the product works 100% of the time to large enterprises so we can close deals. We can't afford weird flakiness in the middle of a POC.
Another factor I rarely see discussed is bus factor. I've been in the industry for over a decade, and I like to be able to go on vacation. It's nice to hand off the pager sometimes. Using established technologies makes it possible to delegate responsibility to the rest of the team, instead of me owning a little rats nest fiefdom of my own design.
The fact is that if 5k/month infra cost for a core part of the service sinks your VC backed startup, you've got bigger problems. Investors gave you a big pile of money to go and get customers _now_. An extra month of runway isn't going to save you.