"Updates"
Sadly the ENTIRE electronics industry can't seem to de-couple "security updates" from "feature creep".
Yes, evolving OS's and applications add features that often require more resources then their older versions and as a result run slower on older hardware.
"# of 'running' applications"
Sorry, IDK WHAT OS your using, each and every application 'loaded' uses up SOME memory and on smaller devices the amount of RAM is not as plentiful as larger platforms. While much of a 'open but idle' application might get 'swapped' all this eats up resources.
While the authors argument about battery life might have merit, the chewing up of resources that could otherwise be used by an application the user WANTS to use out weighs leaving unused applications open.
"more is better"
Sales and marketing types have used this mantra for ages. Seemingly no self respecting purchaser wants 'good enough' or 'just works' but would rather have 'biggest' and hence 'best'. It is all nonsense.
The simple idea that most products that were released +10yrs ago are still more than enough for MOST things is the terror and nightmare's that keeps most sales and marketing folk awake at night.