Ron, if you asked a question similar to this about the HHR and a Lincoln and Jeep, go back and check some of those answers. This is how I see it, the HHR is something you know about per YOUR maintenance schedule, but there are some things that simply wear out. For example, all automatic transmissions need servicing but eventually, the clutches have nothing left, the valves wear out, the seals start leaking, etc, so it's a service NEED to rebuild one if you intend to keep a rig long term. For some people/vehicles/situations that is great, for others its a really grim reality. Have to embrace the idea a slushbox is like a clutch disc that should last longer but...
Luckily with most GM trannys and engines, they aren't difficult to rebuild and normally "worth" what you put into them.
You have to determine if your power steering rack works properly, if the CV/axles are in good shape and if the body can withstand more miles (rust, accidents, whatever), if the struts are shot, any myriad systems and combinations that make you leery of extending the HHR more. You might be at a spot where the decreasing marginal utility of the HHR matches what someone will pay for it and that matches what you want to invest as a down payment for something else.
There are some vehicles that can last longer than others, depending on driving style and type of driving, but let say you own an 89 2500 4x4 that you take out in the woods, tows your boat and trailer and gets loaded down with a bed full of gravel every other weekend, not going to go 400K miles without a lot of upkeep.