Why do you assume the problem won’t exist, when this exact thing happens all the time? Just to name a tiny handful of obvious examples: Oracle, Canonical, GitHub, RedHat, DataStax. Not only could someone add enhancements that justify the price, like several other comments have pointed out here, they could also simply offer support that Defold doesn’t offer, and they could do marketing that Defold doesn’t do. The number of paid products that are equivalent to and/or based on free products is innumerable.

There’s no reason to assume that a paid fork would reduce the number of free Defold users; it can happen, but depends on what is built and offered, and sometimes paid forks are good for the ecosystem and increase the number of overall users.