Assuming Uhuru supports Ruto with the current constitution in 2022. It will mean:
Ruto: Deputy President for 10 years + President for 5 or 10 years
Uhuru: President for 10 years
Ruto will have enjoyed being in and around the top seat for 20 years while Uhuru personally will have had 10 years. Who will have benefitted more from the alliance of the dynamic duo? Who will have gained more in 2027 or 2032? Who will be better placed to ensure their next generation is well positioned.
Remember that Uhuru gave his deputy’s people a few cabinet slots and other positions especially in the first term(kina Rotich and so on). In 2022, Ruto will just tell Uhuru kwaheri because he will have his own deputy to reward with slots. I believe one reason for BBI is for Uhuru to create a bargaining position of PM so that he also enjoy being near power for another 10 years without the pressure of being president. Uhuru will back whoever will give him a better deal and keep him happy. That is why he is playing it safe by keeping both Baba and Ruto guessing.
One mistake Uhuru did in the first term was to associate too much with his deputy so much so that he had to reward his deputy for his support. I think Ruto will learn from this experience and make sure that if he is president, his deputy is a lame duck yes man/woman. He will make sure he does not create a situation whereby he owes his presidency to someone else.