That's true. The higher-ups can say which database to use and hire a bunch of people to set it up. But if the developers ignore them, they have no way of actually determining this. (Which is what's so great about mandates from Higher Up.)
Every few days on pgsql.general mailing lists, there is someone asking for a good Postgresql book (the "High Performance MySQL" kind)
Less books = one more tick mark for Oracle.
If you won't do it, why should anyone else?
My take is that the overhead of maintaining and using Oracle means that you have to cut resources somewhere else. I'm pretty sure a competent DBA and developers can ensure atomicity and integrity using Postgres (instead of Oracle), and have a lot of time left over to create more features in the application.
Its just that RAC is a special technology with no equivalent on other DBs. It is usually placed where you need load balancing on the DB level. It normally has very high availability that is a good fit for mission critical DBs. I hate seeing good technology wasted, but you are right that if Postgres is already there and works well, there is no point in moving anywhere.
Might as well make the best of it.
Not that this is the case everywhere and everytime, but if technical decisions were left solely in the hands of people that lack technical skills, the situation would be even worse that it is now, with all the "enterprise software" floating around and causing trouble directly proportional to their cost.