I'm not getting the "it will cannibalize the sales of their best title" arguement. MK8D has maybe a 10m left in it but MK9 would easily sell over 20m and MK8D won't completely stop selling so from a business perspective there's a greater sales potential in releasing MK9 than just letting MK8D do its thing.
You've awnsered yourself. MK8 may still have about 10m sales worth of R.O.I., why would you waste it?
Plus, if we agree MK9 could potentially be 20m title why would you release it today? You can keep it and "use" it later to boost sales of the next system, right in the early years.
If I had to release a new kart game right now, I would make one with a different flavour, like a Diddy Kong Racing sequel. This way you can meet the demand for a new Kart racing game, while giving the userbase a reason the own both titles and, at the same time, keeping MK9 as the ace for the next console.
Last edited by freebs2 - on 15 February 2020