People, however, (as shminux said) do try kink all the time. It would not be unethical to do a study on people who are already kinky and see if they get kinkier over time.

Anecdotally, they start doing kink, they either decide it isn't for them and stop, or they do get kinkier for a while - because they're exploring what they like and it makes sense to start at the less extreme end of things.

Then they figure out what they like, which is often a range of things at differing levels of 'kinkiness/extremeness', and do that.

I mean, it's almost trivially obvious that compared to the size of the kink community, there is an almost negligible amount of people doing the human equivalent of directly stimulating their pleasure centres to the exclusion of everything else. They tend to make the news. The moderately kinky majority do not.