Rarely would one player be so powerful as to dominate the entire galaxy. More human interaction. Oh yes, there are also planets in those systems. A couple of things need clarification here. By that time, there will be a newer version with an even higher limit.
Common sense offers some clues.
With a limit on ships, however, you wouldn't be able to be in more than a limited number of places at a time. So, large fleets of ships avoid that problem. You'll invariably end up designing all sorts of exporation vessels, mining facilities, warships, and of course there will always be newer models to design as your technology improves. So, it's only fair that you not be unduly constrained in the number of configurations you can design and build. Yes, it does boggle the mind when you think about the prospect of managing hundreds of worlds and ships.