WooingTheStars
For all the Thirds flaws, he is still intelligent and has lived for a long time in the Shinobi world. In his mind, he has always desired peace and would do all he can to protect Konoha. Look into his current mind a bit... He is about to step down as Hokage, Danzo cant touch him as he is still regarded as a Clan Heir. There are rumers of the Akatsuki but its not to the point where they are viewed as a threat. In Hiruzen’s mind the biggest threat is still the other villages. Now, you just aquired a second weapon of mass destruction. A natural war deterant. Konoha is strong with a singular tailed beast, now they have another, and its in someone who knows how to use it. The Mist will flip and demand Konoha to return it, but they were the ones who lost it during the Rin x Kakashi incident. So little will happen from that, plus the Mist isnt in a possition to start a war right now. The whole Fourth Mizukage being control/cival war stuff just happened not too long ago. The Fifth Mizukage took over not long before the Chunin Exams (the one where the third was meant to die) she should still be working tirelessly to reform internal policies and recreate diplomatic relations with other villages. She cant enter into a war with the Leaf at this stage. You have to also remember they lost most of their Kekkei Genkai’s due to the purge/massacre, while Konoha still has their clans. The Sand just suffered a defeat from Konoha and an Alliance was formed. They will only be more inclined to side with Konoha once this news drops. The only issue is the Stone and Cloud. Yet they must be wary of eachother as well. If war breaks it will be a three way deadlock with Konoha no longer having to worry about being pinched from all sides. The main reason Konoha didnt beat the Cloud and Stone was purely due to them being stretched thin across all fronts including the Sand and Mist fronts. They wont do shit, if they do it would be small skirmishes here and there which wont amount to much. He is setting things up the beat he can for Tsunade to take over. Using the Mc’s new possition as in Jin will only lock it down more. Overall its a smart move. The only real downside is the Akatsuki will target him, however, the 3rd doesnt really know how strong they are yet. He has Itachi still in his backpocket because even if Sasuke defected, Itachi is still loyal to the leaf. He wouldnt attack and that gives Hiruzen confidence that the Akatsuki wont be too much of a threat to the leaf as a whole. Thats why he is focused on the villages. Atleast thats my take.
This is pretty much all of the mental process I did before writing those last words. I though of potential threats, pros and cons and reached the conclusion that, however this card was played, it would be more of a pro than a con situation. My main worry on this was Danzo and what he would do, since he is the most immediate threath. But as you said: Ryota is a clan heir who equals a sannin and, if that wasn't enough, he is a Sage and a Jinchuriki too. Danzo can't risk fighting him (and all of the clans as a result) and risk all of his years of planning going to hell just like that.