Give it time, COD4 remastered will see a standalone release.
Releasing it only as a bundle with Infinite Warfare at launch is a trick as transparent as it is tasteless. There is no good reason not to give us the option of buying it separately on Day 1, except that the money-grubbing suits at Activision want to push more copies of their latest entry by attaching it at the hip to a more desirable product. Once they have the holiday season in the bag, they'll fart out a separate release, and anyone who bought IW just to get COD4 will be shafted.
Let's not reward this kind of anti-consumer behaviour.
If you're not too keen on Infinite Warfare, but really want COD4 Remastered, consider waiting just a few months for the all but inevitable non-bundled version. Don't fall for Activision's ploy.