Ultimately I think it's easier to stick to as few platforms as possible to allow your staff to build expertise in optimizing their code and configurations for a platform.
It is so true that the development / maintenance costs, reliability, and performance of a database has much more to do with the level of expertise of the staff than the platform itself.
Just the other day, there was a high profile crisis which got escalated to executive management. The runtime of a legacy ETL process increased from a daily average of 2 hours to over 9 hours after the record volume coming from the client doubled. There was some smack talk about SQL Server limitations and prattle that Oracle could handle the data loads more efficiently. So, I was called in to take a look at the process and put out the fire before it got out of hand. I reduced the runtime duration from 9 hours to 17 seconds simply by analyzing the cost of each query and modifying a single line of SQL code.
As for cost of licensing, the only scalability limitation of SQL Server Standard edition is 16 cores. There are many OLTP databases currently running on Enterprise edition that just as well be run under Standard edition for half or 1/3 the cost using per core licensing.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho