I have the version of that planer that has the knives that can be resharpened. A 733 I believe. The sprocket breaking is a relatively common problem. Not sure there is anything you can do to prevent it and if you got 15 years, it served you reasonably well. Mine broke and I replaced it myself, with the help of YouTube.
Also, mine started to put what looked like snipe on the boards at random intervals. The drive chains had developed some slack. Took a little ciphering, but I figured out the bushings had worn out and there was some play in them. The bushings are made of some sort compressed graphite or some composite that has been impregnated with graphite. Once I got mine out it was obvious they were wallowed out. They are basically a cube with a hole through them. In some the hole was a oval and in others it was still round. Replaced them and all is well.
If your planer is in the shop for the sprocket and it has the same sort of bushings it might be wise to have the shop replace them, "while they are at it". Come to think of it, that might be why the sprocket broke. The wear in the bushing lets the chains develop slack. The slack might be enough to let the chain skip a tooth and shock the sprocket enough to break it. I replaced the bushings and a sprocket and the chain, but I really don't think there was anything wrong with the chain. (I am a cheapskate and fixed it myself, but had to use an impact driver to get one or two of the screws out. The shop might be a good idea)
To bore you even further, there is a YouTube video of a guy repairing his similar planer. It is long and tedious and he fabricates little gadgets to support the chain which he thought was stretched. He ends the first part to wait for parts to come in. In the second video he has had the revelation that the bushings were shot and he did not need the gadgets at all. It was painful to watch, but maybe worth it.