It depends on the application. I can think of one in particular in which the product stays plugged into the AC mains 24/7 and the coincell is needed only to retain certain data in RAM during periods of power outage or when the product is moved from one room to another, which is infrequently. These products typically last a decade or longer without depleting the coincell.
The statement about 10mA draw based on 200uA/MHz and maximum clock rate of 48MHz not being useful is pretty misleading. Quite a lot can be done with microcontrollers running at less than their maximum clock speeds, although wasteful programming and inefficient compliers can certainly result in the requirement to run at full speed. Efficient design practices generally payoff in less power, less rfi, less heat, and possibly smaller size/less weight.
The information about the internal resistance change is interesting. We saw a similar issue when looking for supercapacitors to power SSDs during power interruptions.
It really depends on the application. This month I experienced two instances of battery failure in devices where sleep current largely defines the battery life. In both cases, a CR2025 is used to drive a micro that sleeps until a key is pressed.
The first instance, a DVD player remote control, died at about a year old. The control was found under some books, and had been there a while, likely pressing buttons the whole time. If that hadn't happened, I suspect the battery might have outlasted the player.
The second was the key fob transmitter on a 10 year old car. I've had the car since new, so I know the battery had never been replaced. The dead battery showed a few millivolts, and the new battery stopped working after a couple of days. I measured the sleep current, and it was way too high to support a 10-year battery life. Scrubbing the PCB with flux remover dropped the sleep current to a much more reasonable level. I was going to replace the battery in the second fob (which gets more use) but I think I'll wait until it (or the car) dies just to see what the lifetime is.
I think in both of these cases, the battery would normally have outlived the device, or at least the original owner, making it essentially maintenance-free to the original customer.