Welcome to the Forum:
I have one of each model Kirby, right here at the Vacuum Cleaner Museum in St. James, Missouri. I also have some of the gold plated models that were sales prizes.
Throughout history, Kirby has always been "late to the game". Disposable bags were invented in 1920, to eliminate housekeeping's most disagreeable job (emptying the cloth vacuum bag). Kirby wouldn't put one on their machine until 1979.
On-board attachments were invented in the mid 1980's to make using an upright easier and making attachment use more convenient. Kirby still hasn't figured out how to do this yet. "Late to the game".
Kirby inhales the dirt through the suction fan. Fan damage is high on Kirby with hose use. The fan is running at double speed, and a rock or coin inhaled through the hose hits that spinning fan very hard. While having a vulnerable fan helps get the rug cleaner (more airflow right at the carpet), it's no good for attachment use where damage is likely (and expensive to repair).
Kirby still uses a vulnerable belt, twisted into a figure 8, right in the middle of the dirt path. All the dirt inhaled must pass over the belt, meaning the belt has a much shorter life. If the belt were in its own channel, so dirt never touches it, the belt would last longer.
Kirby bags are expensive (2 Hepa bags in a package for $10). Ever throw a $10 bill into a trash can and walk away? Me neither.
The Kirby cleans very well. They form a good seal with the carpet. They have a great height adjustment mechanism. The brush can be adjusted to compensate for bristle wear. They clean large, open areas much better than small crowded rooms.
At one time, most vacuums looked like Kirby does today. The other brands evolved, became modern, became what customers wanted. Kirby doesn't have to evolve. They are sold by door to door salesmen who work on commission. The job of the salesman two fold - one is to get into your door - and one is to make you unhappy with your present cleaner. No one wakes up in the morning thinking "I'm going to buy a Kirby today". A Kirby isn't a vacuum you shop for, comparing it in-store to other machines. Rather, you are led into an unwise purchase that in most cases has to be financed (can you say 23% interest?). The customer never sees the salesman again, and are lucky to have a Kirby sales office remain open in their town for any length of time.
There are always exceptions. Some Kirby salesmen are honest people, who run an honest business - and have for years. They provide service and repairs and their customers are kept happy. Sadly, for the vast majority of Kirby owners, they regret their impulsive decision to buy one.
On the other hand, vacuum collectors tend to love Kirby due to the retro look, the high performance, and the snob appeal of the machine. Even I run a Kirby at least once a week.