Are you wanting reassurances, guarantees or strategies to prevent this fear of yours happening?
Sorry, but there are no guarantees, not about this, not about much in life actually, except maybe having your toaster replaced if it's faulty. I have heard of it happening although I wasn't close enough to be able to comment on the ins and outs of the situation.
Reassurances: It isn't very common. Red pepper said it and my partner feels exactly the same; polypeople don't stop loving in the same way that mono people do. Unless they have been very hurt they generally don't stop loving they just add loves, their hearts have a capacity to keep expanding to include. The concept of moving on to greener pastures is very much a mono concept.
Personally what I have found being in an essentially poly/mono relationship (me being the mono)is that I have grown more in the past three years than in the 28 years I was with my supposedly mono husband. Sure that might be due to other things but if you're open-minded and just relax into it, being around polyamorous people seems to make you more interesting.