Obito’s body turned to dust with Hshirama’s. as you know, with Naruto 689 Kyubi inside naruto, that Naruto’s chakra used Kyubi’s as it leaked and it basically became one chakra. Therefore, NObito would still have those abilities at death.
You assume it’s a tech that needs experience to do. Just because it took Sasuke, who just recently awakened his MS (he’s likely had it less than half a year in the manga time where as Obito had MS since Sasuke was born and Kakashi has had it for about a couple of years), some time to reach perfect doesn’t mean it would be the same for someone else. Plus, this is clearly a completely different situation. Itachi didn’t have a perfect Susanoo because he barely ever used his MS. He had low chakra and an illness.
The point is Kakashi has spent a much longer time mastering his eye than Sasuke. As did Obito. Both eyes were already in full use because each eye had specific abilities, separately that kakashi and Obito mastered. Once they were able to combine eyes, it was able to fully manifest Susanoo. Seems like an open and shut case.
If you understand how the technique works and you’re been using MS for double digit years like obito, as well as using Rinnegan, I think you can be given the benefit of the doubt on using your new techniques that you didn’t initially have access to. Did mAdara not pop in Obito’s eye and Kamui like had been doing it since he was a kid? lol
Mastering the ability in that eye means he learned how to use that eye. Kakshi mastered the other eye. Between the both of them, they had fully mastered eyes. Only lacked the ability to awaken the other techniques due to the eyes being separate. Problem solved now.
They can awaken it in one go because they just did. Itachi, Madara and Sasuke don’t mean anything because they aren’t in this exceptional circumstance that kakashi and Obito are in. We’ve already seen different Uchiha be better with different MS techniques and some have their own unique MS abilities. Again it’s an open and shut case to me. People just are too closed-minded and like to rage without actually thinking about what they’re upset about.