There are many opposing forces and dynamics. There is cost of having many servers and the cost of having just one. I think this question may telate further than just database? I may be misunderstanding, but it does relate to a systemic misunderstanding that is out there re costs of tangibles vs abstract
Typically the obvious costs are easy to understand.
Abstract costs are harder to quantify and thus harder to understand. TechnicalDebt, cost of errors, cost of stress, burden on developers, effects of change, regression testing, impact of downtime and so on are harder to explain.
Environments are usually separated for data and/or purpose. Each environment has a different function. The rate of change on a system, ie. how often it will be updated, what sort of changes and effects of change, are all considered.
We use different environments to trivialise change.
We use different environments, so we offer robustness and certainty of the environment that has not changed.
We use environments to consider the effects of a change.
We use environments to reduce the costs involved with change.
it costs a lot to test and stabilize a system
You create environments to secure the investment that was made on the stable environment.
You are never too small a team to adhere to pragmatic, cost saving, diligent and proven patterns of process.
Using one environment for everything is like storing all your photos on one harddrive, you can do it, but you gonna regret it.
some people need proof
I have been in many situations dealing with clients or others who dont appreciate the costs of ensuring robustness and following best practices. I would suggest you put together some examples of real cases where the costs are clearly defined.