Hopefully this limit can be raised/removed or we can hear about why there is a limit on the number of pieces.
There is exactly 0 chance that the limit would be removed in whole, for obvious reasons that have to do with the features' functionality.
As expected, the actual limits for the feature will be balanced around a number of different factors (file-size, feasibility, functionality, performance impedance etc etc).
To give a quick example. This game is designed so that it can be played by all players that
A. Have paid for the game.
B. Have a computer with the minimum requirements (and have set the graphic/simulation settings accordingly for their computers).
Now, imagine a player that possesses a monster PC rig making a gorgeous cathedral building, complete with interiors that actually make it a food plaza. The Cathedral has a wonderful, absolutely stunning roof, made from about 1200 wooden signposts stacked in every possible direction.
He certainly wants to share the building with his friends, and other Planet Coaster players. So he makes a blueprint out of it, puts an annotation in the description saying that it is a heavy asset - resource wise - and publishes it.
There will be many, many players with middling laptops that will download it without reading or paying true attention to that annotation. Especially since the playerbase in a theme park CMS game has a lot of children too. Their game will lag, they will remove the building, go to the players blueprint page in the workshop and downvote it - and him - massively. Or blame performance of the game to Frontier, open bug reports, trash the game in the Steam forums etc etc.
If you are thinking that something like this cannot happen, I assure you it will. Whatever the end limit might be. So, there are a lot of possible scenarios where an actual limit is prudent to have. And that limit will be determined by the developers, not us. We will have to play around it, according to the game rules then, not ask for the rules to change for our own purposes.