They already do, somewhat. I have unlimited 4g LTE data on my phone with Verizon now. I connect my 4g LTE Iphone to my TV with an HDMI adapter cable and I can stream as much as I want. I can also connect my Netgear wireless repeater/router wirelessly to the hotspot on my phone and run my home network off it with no additional cost. Recently dumped DSL in favor of LTE.
Edit: I should add that Comcast won't let you do "screen mirroring" of your Xfinity streaming channels from phone to TV. Netflix, MLB.tv, etc do allow it. In those circumstances, for example ESPN, I use their native app on my phone to mirror to TV.