First of all, engine rewriting would take years, you'd have to rewrite everything, key word : Everything
I suppose its not inherently obvious that everyone and their mothers would be aware that an engine rewrite would take a long time, so thank you for keeping me up to date in that department, I don't know what I'd do if I just went around saying things without thinking about them first and didn't have anyone to tell me these sorts of things. So again, thanks <3
Most of those limitations are a result of the stupidly small packet size limit of 1 kibibit (128 bytes) badspot has for whatever reason profusely refused to raise.
For example, vehicle updates get priority over player updates in networking, so the number of vehicles has to be limited so that a rogue player can't spawn 100 vehicles and all of a sudden nobody can see anyone else moving.
Whose idea was it to make vehicles more important then players, I can't really think of a reason why that should be the case (except maybe that it's easier to update vehicles then players?); but I can't really say I have enough experience making multiplayer games to answer that either so... w/e
If a host has made the limit that high, and a player has come online and done this, then the host is also partly responsible for not being able to moderate a server properly.
Exactly the reason why I question the existence of the 20 vehicle limit. Granted I can't think of very many scenarios in which you would need more than 20 vehicles, but I've definitely hosted fort wars servers where having that would have been a nice improvement. That being said in the current state of Blockland's vehicle physics, vehicles themselves aren't very usable.