Good news, and a good decision by the commission.
The merger of Orange/T-Mobile was fine imo - at the time we had 5 operators (Orange, Voda, O2, Three, T-Mobile), and T-Mobile were a relatively minor player in the UK who had been losing market share for years. So dropping down to 4, all of similar clout (albeit with different market focuses) wasn't really an issue. It proved positive for consumers too, and EE has worked out really well as a network provider.
Merging O2 and Three would have been a step too far in my opinion. It would have taken us down to 3 operators which would really be pushing it. Very happy they have stopped this, and hopefully it will help to continue the price wars we've seen over recent years. My mobile bills have dropped hugely over the past 8 years and by keeping competition alive, hopefully they will continue to do so (at least in terms of value for money).
As to why the UK competition authorities didn't step in first, well thats a whole different question. I'm not really sure and tbh I don't really care