I think you may have misunderstood my phrase "the will to use them". This is as opposed to the desire to push it in higher gears. Generally, as long as the gearing is low enough for a high (which means anywhere upwards of 80 something, but could mean 100+, depending on the person), cadence, bicycling is easier on the knees than walking, which in itself is easier than running or jogging.
Beyond that I would strongly recommend against using SPD-style clipless pedals. I've not used Speedplay Frogs, but I understand that they may be easy enough to get in and out of as to cause no knee stress. But I'd want to verify that. I personally have SPDs, and had to switch one of them out for a month or two when recovering from a knee injury a couple of years back.
As far as weight being the culprit for knee failure, sometimes. Yes most very heavy folks have bad knees. It is also the case that a disproportionately high fraction of tall ones do, without being overweight. As I mentioned, I'm planning/hoping to avoid knee replacement for many years to come, but being tall, and not at all overweight (BMI 21), I do have issues. Exercise helps. Certain activities make it worse. It has much more to do with my height than my weight.