That's like saying, I have a 1TB hard disk.
Now I have less files, so I can save money by not upgrading.
Sure, you might not be forced to upgrade due to space constraints.
But harddisks are mechanical devices - they degrade over time (just like trains, roads, plumbing etc.). If you don't replace your harddisks for 10 years, because you somehow manage to delete a bunch of files, and not replace them ....well, good luck trying to access your data.
Also, technology is constantly evolving - and trite as it may sound, if you're not going forwards, you're going backwards.
Have you tried using, oh, say, a Quantum fireball drive lately, to run a modern operating system or modern applications?
I mean, apart from the incompatibility issues, even if you got it running, and even if you managed to fit it on there, it just wouldn't be able to keep up.
So no, having a falling population doesn't mean you can just sit and let your infrastructure run into the ground.
Problem with it is that it ignores that physical things don't follow standard accounting depreciation schedules. Like I mentioned, you can't make the assumption that physical resources like energy, labor, and materials flow through the economy like intangible 'money'
The problem with your equation is that real things don't degrade by X percent per year. Or more that the utility tends not to. For maintenance you usually have a fix part that is based on time and a variable part that depends on use. And typically the the cost of depreciation occurs mostly at the end of life when you need to expend real resources to replace it. However if you don't need to replace it then what? Bonus.