I can understand the flawed line of thinking that leads to not taking salaries and time into consideration when making comments like "why not just do it yourself?", but it's interesting how rampant it is. I've worked with people who are like, "To avoid paying some company $999/year, I'm going to invest $1,500 of my time setting up a free version running on hardware that has a smaller recurring cost (but still does have a cost). And let's just not worry about how much of my time might be required to support this once it's up an running."
My last gig had an MSSQL database component that pushed $700,000 a year.
It wasn't _that_ big but it was pushing the upper published limits of what you could do with an MSSQL RDS. One day replication stopped working, and amazon business support of whatever couldn't resolve it.
I've been around the block and have continuously come to the conclusion that it's usually better to have the skills to run them yourself, and then it's just a balance of whether or not you're time or cash poor.
It also depends a lot on your scale. My last company was paying $50/month for a hosted database (4gb RAM), we upgraded to one with 8GB RAM for $200/month that was likely to be sufficient for the foreseeable future. That was expensive for what we were getting, but it certainly wasn’t worth our time or effort to build out our own.
Any company that has an engineer is going to most likely have them full-time. They either work on standing up a database and keeping it alive some of the time, or they watch their business go from $100/mo -> $1k/mo -> $10k/mo for a PaaS database, no?
That is assuming you're successful (or at least have a busy storage tier.)
I think the people who are doing this calculation wrong are mostly confusing and conflating those two outcomes. Just because your database servers are very busy, does not mean your product achieved some commercial success. You can't just pay extra for the full-service treatment and expect to receive a commensurate output in value,
(and the people who won't pay for a real person to own their databases full-time are likely in a circular Venn diagram with people who won't pay attention to optimizing their database queries, so your point can probably stand unmodified... except there is an important case where the PaaS provides value, and it's nearest to the starting point of practically everyone.)
If you look at every dollar spent on IT/databases as a sunk cost which cannot be recovered from future recurring expenses, it's very bleak indeed to put pennies on top of pennies and pile them higher every day... or you could pay attention to the signal that SaaS is providing every month (the bill for usage), and ramp up the attention paid to optimizing queries before it gets too late in terms of dollars and cents? The dollar impact of slow queries cannot be understated.
But some business ideas will also not ever make it that far. You could spend the money on DBA salary or DIY and never know how much it was really being wasted, comparatively, if you didn't ever get a handle on your own usage metrics.