I'd rather them just block these things like they did up until the early 2000s ... but it's not unreasonable for them to break up the government granted monopoly on spectrums if they do allow mergers.
It concluded that markets with 4 or more providers were far better served (price, quality, coverage, etc) than those with 3 (which effectively is what the US will have now).
The UK blocked the merger of 3 and O2 on these grounds, as it would leave only 3 providers.
But expecting competition from the cable companies is a bridge too far. The cable companies are the most anti customer friendly companies in the United States.
My assumption so far is that things are shifting such that LTE/5G are largely replacing CDMA, US govt would still prevent a GSM monopoly situation.
Cable companies are competing though - comcast started in 2014 'xfinity wifi' rollout via leased modem then got into mobile in 2017. If I'm reading https://www.xfinity.com/mobile/ correctly, it's cost-competitive ($45/mo unlimited compared to $70/mo on tmobile) - and works with the iphone myself and many US consumers already have.
I'm guessing verizon may already be doing something similar with fios modems.
I don't see that happening. Xfinity Mobile is not a competitive product. It's just a Verizon MVNO. It can't seriously compete against Verizon, because it is Verizon.