This may be true, but it also makes the common assumption that your time (or the time of your team) is somehow worth $0 dollars. But, I imagine that you would be the first to say that your time is worth much more than $0 dollars.
Put another way, hardware stores sell paint and paint supplies all day. Buying those and painting a room yourself is certainly cheaper, but it assumes you have the extra time to paint and don't mind you being the laborer. Hiring a painter costs more, but saves you a ton of time, energy, and elevates the level of expertise you are bringing to the job.
Getting started is one thing. The ease of how you maintain and grow is another.
I'm not sure ops time is even the one to worry about. Most things I've learned I've learned the hard way. So a server going berserk isn't fun, but it's not like I just wasted either time or money on it. I learned something. I better understand my execution environment.
Downtime, on the other hand, is just lost revenue opportunity.
I certainly do, and I can tell you the amount of time I've spent managing servers over the past year has cost us less than running our entire production stack on Heroku would have done. So has the odd bit of downtime we've encountered because I've not got the same level of experience as Heroku's ops team.
Man, I need to go break the news to our customers that are successfully inserting millions of documents per hour (and their apps running nicely ... probably an app you have used) to inform them that, in all reality, they are "failing miserably."
Considering MongoDB has only been around for the "last few years", I think it is maturing well. You are certainly free to call it the PHP of Databases, but there are a lot of developers that have learned to use it and are quite happy with it. Seems like most on Hacker News spend more time telling them why they are stupid than actually understanding the tool.
(Also, some guy by the name of Zuckerberg made a TON of money with PHP.)
Also, the tool is still in its toddler years ... and it is constantly compared to tools that have been around years longer (PG/MySQL). Comparing it to other NoSQL databases is just as silly (Riak, Cassandra) because they are completely different, have different constraints and are solving different problems.
There aren't better choices ... there are just different tools that are better for different jobs. It would be like looking at your toolbox and chunking everything but your saw or your hammer. Too many people are looking for the "technology to rule them all", but that doesn't (and shouldn't) exist.