Don't want someone else to continue on the topic of my thesis
Congratulations on the successful defence of your thesis. If at all possible, take some time to relax.

I understand your concern, but you have no real reason to worry. In general, there is always room for more research on any given topic. Now, anyone who wishes to advance your work will either be more senior than yourself or working for someone more senior than yourself. It will be their responsibility to decide if they want to invest their time and energy in the project.

If you are ever contacted about your thesis, then you can explain why you are less optimistic today, than you were in the past. It is very likely that the other person has some fresh ideas and together you may move forward.