Here are a few ways to look at it.
MK8 is online so the community will shrink with time. If you get it now, you can play while it is at is most active period.
Mario Odyssey is single-player which means the game will be the same as it is now 1, 2, 3, 4, 5, even 100 years down the line.
At the same time, Mario Kart 8 is not the Switch's Mario Kart. It is a Wii U Mario Kart put on the system, so it would have a decent library at its start. A Mario Kart 9 for the Switch is sure to come.
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