The 10 is a huge improvement over the M8. Negatives are the downgraded speakers and the dimmer, polarized screen. Pluses are the dramatically improved headphone audio, speed, and camera. I wouldn't think twice about it. I am using Bluetooth speakers and headphones now so the speakers aren't so important to me anymore, and for most use cases the screen is more enjoyable, at least to me. It's very sharp. I see myself hanging onto the 10 far into the future as a dedicated portable hi-fi audio rig.
One reason makers might be moving to glass backs is to improve signal strength and to avoid dealing with the problems of engineering antennae embedded into metal cases. Cynically, it's probably also to lower costs (Didn't HTC say it took 3 hours to manufacture the metal body of the 10?) and to raise the possibility of future revenue from each device as more repairs will be needed. Likewise, the removal of the headphone jack opens up more revenue possibilities, too. With Bluetooth 5 arriving, wireless hi-fi becomes possible, and if you're into high-end earphones, the headphone manufacturers won't mind selling you expensive headphones with batteries that will wear out over time.