My Moto Z² Force is only 10 months old [because it was a new replacement for a defective one]. I love mine, and have no intention of ditching it any time soon. But I'm not you!
I always buy my phones directly from Motorola, so they're already paid for and that isn't a consideration. If I had the 'free upgrade' option, but it was for a not-very-new model, I'm not sure what I'd do.

I guess it would depend on whether its specs were that much better than my current phone.
And if you do the 'free upgrade' thing through your carrier, does that lock you in to another 2-year contract with them? That's something to factor in. If you're happy with your carrier--its service, its coverage, its support, its cost, etc.--and want to stick with them, being locked in isn't a big problem.
You might try an old, tried-but-true method of deciding: write down the pros and cons of keeping/upgrading. Just make two columns on a piece of paper, one for pros the other for cons. When you're done, see which side is longer and go from there!