Posting in a thread has a much lower effect compared to pressing the "bump" button. As such, you should not worry about "bumping" a thread when you post in it.
One post from someone who earned a lot of Merit in the past year is enough to put a topic close to the top.
Temporarly.
Although not explicitly part of the merit system, those with more merit tend to have more credibility, so if someone is being critical of a project with a lot of merit, the thread will be close to the top of the page (temporarily), but your post will also have more credibility than those defending the project if those defending the project does not have much merit.
Ignoring the above, the bump system was not designed to forever keep scam projects away from the top of the first page. It was created to prevent spammers from excessively bumping a thread to the top.