That's your other mistaken assumption, no scientist worth their salt would think they have got it all figured out. In fact, their whole job is to question everything. Science is continuing process about understanding the Universe, one that never stops. There would be no General Relativity if Einstein had thought Newton's laws were the ultimate truth. That doesn't mean Newton was wrong - of course he wasn't - it's just that there may always be a more complete theory, and that's what scientists are always looking for.
Till then, scientists will hold the theory that explains the most and is most intensively peer-reviewed most to the highest standard.
Indeed, it's the media that goes around brandishing things as "facts". That's not a problem with the scientific method.