Unless someone wants to trade for your kitty right away on the trade thread? Part of the fun is that they go up their 3 part evolutionary branch and then switch branches entirely after that! Yes, taking care of other ryus is helpful and will help increase your luck, which in turn helps determine things like getting more scenes and paints and whether you continue up the evo brach you're on. Kitty turns into a sabre tooth (something like that?) then to a lion for example.
1500 scoops earns you a fancy userbar! There's another ryu related bar for "dedicated ryusitter." Stats aren't officially out on what it takes to get it but both consistency and quantity very likely go in. Ryu caretaking is a fun side hobby. It takes a village.