First of all: everything can be cracked. But that's not the point of LimeLM:
What is software licensing?
Software licensing allows you to get paid for each copy of your software. The types of software licensing come in a few general forms which I'll be covering in the next section. With few exceptions the hardware-locked licensing is best for businesses because it allows you to have absolute control over where your software is installed.
The point of licensing isn't to stop crackers from cracking your software. The point of licensing is to increase your revenue by preventing casual piracy (using serials over and over again). There is real money to be made by stopping casual piracy.
Unfortunately there are dishonest licensing companies that claim they can't be cracked. They're selling snake oil.
Am I right by saying, since the TurboActivate.dll is the same for all your customers, once it is cracked, all apps from all users using that same dll is cracked?
No, just use a hash of the TurboActivate file, hardcode that in your app, and if the hash doesn't match the file on disk then the file has been modified. See: Re: Replace TurbleActive.dll with a fake one?.
That will stop simple replacing of the file.