First T-Mobile US brought competition to AT&T and Verizon Communications. Now it may be bringing it to Comcast.

T-Mobile’s recent offer of free wireless data for video from streaming services such as HBO Now, Hulu, Netflix and Showtime could theoretically help subscribers cut the cord, not just on pay TV, but also on wireline broadband. For now, the quality isn’t up to cable, and it might not be economical for T-Mobile if too many subscribers use the perk.

But...