That it is actually scarce. I understood that in a vacuum Bitcoin was hard-capped at 21 million, but I didn't see why an unlimited number of almost perfect substitutes couldn't be generated. That would make the class of Bitcoin-like tokens functionally unlimited.

Technologically, that concern is somewhat valid, but it doesn't really make economic sense. One of the Bitcoin-like tokens will be preferred to all the others, because network effects will make it generally more valuable, and that will drive the value of any similar good to zero.