The first cost is from a martial arts perspective. It's quite simple: if I don’t train to deal with a technique, there is no guarantee that if it's used on me I will know how to react. My reactions to wrestling style takedowns have dramatically deteriorated. I didn’t realize this till I went to a Brazilian jiujitsu workout and had the opportunity to train with Renan Borges. Renan is an accomplished Brazilian jiujitsu blackbelt. After being introduced as a nationally ranked judoka and a training partner for members of the US Olympic team, you can imagine my embarrassment when Renan nailed me with a low single leg takedown out the gate. (To his credit, it was a great shot) In my transition to Brazilian jiujitsu competition, I’ve had to relearn wrestling. To keep this in context I wrestled for a two time state championship team in highschool, and grew up defending wrestling style takedowns in my judo career. Just imagine how difficult it will be for a generation of pure judoka to transition to a MMA or self-defense situation against a wrestler.
The second cost is paid not by the athletes new to the sport, but by athletes who have been in the sport for a long time. You have stranded skillsets. If an athlete specialized before in a powerful pickup game, they won’t have an opportunity to use it again in a competition setting. This means potentially hundreds of hours of wasted time and significant frustration. I only lost one technique in the first phase of the rule change (an off the grip firemans carry). Friends of mine lost most of their repertoire.
The final cost is a little bit more abstract and will matter to differently to different people. In a sense when you think about executing a technique, there's a memory attached to it. You might have very clear memories of having used it or defending it. When a technique is accessed, at least for me all of the memories associated with that technique (both good and bad) are accessed as well. When I hit an uchimata I remember training camps in upstate New York. When I throw osoto gari, I remember one of my old coaches yelling at the top of lungs for me to attack with it. When you ban a technique, you reduce the opportunity for these memories to be accessed. Everyone will put different values to these memories, but for me they matter. I understand when a technique is banned for safety reasons, but when it is banned for sport aesthetics I wonder if the benefits outweigh the costs.