The consensus was and is that the higher loads and deeper discharge demanded of a battery from E/S coupled with less than optimal re charge cycle of the bikes charging system impose more electro chemical wear and tear on a battery than it would be subject to in K/S only service. The repetitive discharge/ charge or undercharge cycles tax a cumulative toll.
Fair enough, and useful information.
There is another important factor to add to the mix. One reason a battery will last longer on a k/s-only bike is because it doesn't need to be as healthy in order to start the bike. It could have a high internal resistance and very small capacity and
still start the bike, because it only has to power the ignition system.
The battery on the e/s bike has to be able to provide a hundred amps or so, which is much more demanding.
My point is that even if the batteries degraded at exactly the same rate in the k/s and e/s bikes, it would come up short on the e/s bike sooner because of the greater demands upon it.
Personally I've yet to be convinced that electric starting itself plays a significant part in wearing out the battery (unless the bike is a bad starter, of course), but I'm open-minded to this and happy to change my mind in the light of evidence.
Meanwhile, though, I wouldn't mind if I had to change the battery every three years if it meant I had the convenience of electric start.