That's very true of Call of Duty but I think BF4 and Hardline could run well together and give gamers a vast choice of two very different games with similar core mechanics.
By Christmas, you've seen it all in a COD game with only the DLC to retain any lingering longevity. You've unlocked all the guns, attachments and perks and the quick-fire matches of 6v6 just means a continuous cycle of the same old stuff. I usually tire of the current COD game long before summer as the hype for the next one grows as the leaks reveal.
Battlefield 4 has over 130 weapons to unlock, tonnes of assignments, medals and matches on a huge scale on a variety of modes. There is so much meat and depth to this game, it is just massive and despite the problems; the infrastructure will keep players engaged for a very long time. I spent a couple of years on BF3 and I'll probably do the same with 4 as there is still so much to do and see. The backbone of this game is immense and that's why I think people will still maintain interest even if there is a sister game out at the same time. BF4 accompanied by Hardline as an alternative sounds pretty good.