Indeed. But BT will be asking for minimum line speed requirements before signing anyone up to the new services. Just like they do now with BT Vision. The minimum line speed requirement will be 26Mbps. And at that speed, there will be no impact on others home internet usage.
I don't see how they can guarantee that unless they can maintain completely separate backhaul paths from wherever the linear channel bandwidth is measured and controlled (cabinet/exchange/wherever). To take it to an extreme, if all or most Infinity users connected to a cabinet or exchange were to be watching one HD linear channel and recording another at the same time (hypothetically the World Cup final/Champions League final etc.) there would be an obvious risk that they'd have to take a big chunk of bandwidth from all ordinary Internet users in the area, to maintain linear channel quality, and that it would make a big dent in everyone's Internet speeds at the time.
I'm sure they will plan for this possibility and I just hope they get it right, I've got well used to my 75Mbps rock-solid 24/7 internet speeds now and I quite like it!