Putting up more towers won't solve the problem RF goes everywhere the cellular spectrum is limited and with WIFI creeping into GSM bands (and vice versa) it's even more saturated now.
Put more towers and your signal to noise ratio goes down the toilet.
As for transmission power well this is regulated T-Mobile can't just crank up the power.
Not to mention if they start spreading out towers like mushrooms the "health and safety" nuts will pop up after them claiming that towers give you cancer and they should be removed.
Comcast and any other ISP can pretty much increase it's bandwidth indefinitely it's only real limitation is how much room it has under the ground and hint that's allot.
And considering that people go crazy when cell providers try to spread Wifi around and push people using data (and even voice some times) to non cellular wireless internet this is pretty much a mess the consumers have made for themselves.
If you live in a metropolitan area you should petition your mobile provider to expand it's Wifi network and even partner with a residential wifi sharing providers to give you more coverage.
Cellular bandwidth is becoming more and more expensive and until h265 becomes a streaming standard don't expect to be streaming 1080p on your mobile devices in a dense area.
I live in London and I use maybe 500MB of 4G a month and that's because everywhere in my home, tube stations, work, and commercial areas there is a hotspot which is either provided by my carrier or my carrier has partnership with and I am getting very good download speeds all the way (I've cached the entire map of London on Google maps which was about 900mb yesterday in about 4min while waiting for the tube without any hickups).