While true, that's not going to motivate anybody. Memory is not a very scarce resource, we're making more all the time, we can continue to make more, and it's reusable.
Moreover, memory that isn't being used is almost completely useless - the only thing it does is act as disk cache.
Better to use memory than CPU, as the latter actually consumes power (leading to climate change if not powered using renewables etc.) - although even better to use less of both.
A more effective argument would be to look at the number of users on low-end devices, and point out that, the more memory you use, the less these users are able to run your applications.
Not that I'm expecting companies like Slack to care - they design for the high-end user, and if your company forces Slack on you and you have an older device, there's nothing you can do, and they exploit that.
Reason for that is that DRAM must be refreshed 24 hours a day, while the CPU is sleeping a lot of the time, even if the device is in use.
Programmers working on products with a billion users should realize that every kilobyte they save decreases world-wide memory usage by a terabyte.