It was nice while it lasted, although I'm not sure that many people were abusing tethering (or even using it that much, if at all) for Three to need to do this.
My wife is on the One Plan and hasn't tethered once, so clearly she won't have a problem changing - but it's a lot of negative PR for the network (and ammunition for the Three haters) in order to remove something that should have been manageable with TrafficSense.
At least the unlimited data on the handset remains, which still gives Three an edge - but for mobile broadband, I'll probably just start using my EE mobile broadband account more. With double speed, and the option of getting a Cat 6 LTE Mi-Fi at some point in the future, I'll clearly get far quicker speeds too.
I suspect a fair few people who now know 'it's over' will start to consider paying a bit more to go to EE, knowing the network coverage will be essentially identical, but with far more 4G and with double speed/4G+ too.
When EU roaming charges are axed, that's one more thing Three 'loses' as a benefit.
I hope Three knows what it's doing. It does need to make more money, but really can't afford to lose customers.