I agree that multiple kettlebells is the way to go if the kettlebells are all the same size regardless of weight, as is the case with pro-grade kettlebells (as mentioned above) and other styles–such as the Valery Fedorenko performance bells. But the real issue is dimension. I would have no problem with adjustable weight kettlebells if they are able to make them adjustable in weight and still maintain a consistent dimension. But that would be a rare find.
Albeit, in answering the question on the table, there is some consideration for the type of training you do with the kettlebell. Many people are adapting their use for a wide range of exercises, many departing from exercises that utilize the true benefits of a kettlebell’s dynamic movement. To each their own, but they may be able to do what they want with any kind of kettlebell.
All that aside, to get the full benefits of kettlebells, I suggest fixed dimensions and kettlebell-specific exercises. Otherwise, quality of reps will be compromised, and such a compromise can easily lead to injury.