Can't Thanos do the exact same thing, now?
Sure, but there's no guarantee that Thanos would discover an outcome that would lead to his demise.
For all we know, if Dr. Strange would have continued his search through the future, then, he could have discovered another couple, or even couple hundred, different paths that lead to beating Thanos. Conversely, it's also possible that he'd never discover another single one..
Similarly, if Thanos were to look into the future, then, it's easily possible that he might not discover a future that leads to his doom until his 4 billionth trace of the future; maybe even 4 trillionth!
Given that there are literally an infinite number of "branches" that occur when searching through the future, really, Dr. Strange was extremely lucky to find a winning future in just 14 million searches.
At the same time, perhaps Thanos just doesn't have a motivation to do such a thing -- he's just obtained the six most powerful objects in the universe, is now the most powerful being himself, and, has just accomplished his ultimate goal.
My point is that is can Thanos override Dr. Strange's "Future"?
If he's able to find the exact same future that Dr. Strange saw, then, maybe it's possible, but maybe it's not.
This kind of topic usually invokes paradoxical arguments, etc., so I won't get into all that, but, in general, perhaps some futures can't be changed, regardless of a character's efforts to avoid it?
A classic example of this kind of story (where the character is aware of their future, makes an active attempt to avoid it, but by doing so actually fulfills their destiny) is the Greek tragedy of Oedipus Rex.