The "finisher argument" that gets rolled out is "product lifetime costs". It goes like this.
Open Source Product A will always be your responsibility, which will take X hours off the team's time forever. Vendor Product B will be maintained from the vendor side, thus removing that wildcard of X. Ergo, over the "lifetime" of the product, B will have lower costs than A.
Since this argument has dollars in it, it always wins, and everyone makes smug faces.
I personally think it's almost completely ridiculous to evaluate software with anything called "lifetime costs", and honestly, it's getting pretty close to ridiculous to cite that metric for any complex system, physical or not. But that's just me[1].
I'd be interested to hear what the general readership's opinion is on whether or not the "Lifetime Costs" argument holds water, and why.
[1] In the software case, what's a "Product" "Lifetime"? Is that "the software in its current precise configuration", because then we're talking a lifetime of picoseconds. Or is Product defined so broadly that virtually any software of that class can be called "the Product", in which case you're stretching my credulity to say you're capable of supporting this notion of "Product" for N arbitrary period. I don't think you will make any money doing that. Either way . . when it comes to "lifetime costs" for physical complex systems, there's a supplementary argument, out of scope for this discussion, but still applicable in heavy industry.