the caution i would add is, you could try to play nice & the employer gets pissed anyway and say “you know what, you can leave today!”. one must prepare contingency plans on both sides.
if you want someone who will never quit, hire an independent consultant.
I can think of many reasons why an independent consultant would quit: abusive or annoying relationship with customer (e.g., customer wants lots of free revisions), bills don't get paid promptly, can negotiate a better rate elsewhere, bored with the current gig, etc.
You should go honest and transparent first, but if people play games then you have to protect yourself. However, my theory is that consultants are less prone to the crazy political storms that engulf full-time employees and give them insomnia.
Of course it’s bad for a consultant to get the reputation of being unreliable so there’s some moderation on the practice.