Then Meta would help everyone cope with the extra workload ... then help some more with a few changes .... then offer some new features ... then help with increased usage ... then offer more features ... then push out the smaller instances and take over everything ... then wall off ActivityPub ... then start charging people and advertisers .. then make billions ... then watch users rebel and start a new system and repeat it all again in 10 or 20 years.
Pretty sure it'd only increase traffic if folks on a fediverse server followed someone on Threads. So unless there's a sudden gigantic flux of fediverse users following Threads users, it shouldn't have a huge near term impact.