Madara was nerf as JJ so yes he could have easily killed everyone who fought him during the war arc but you can't say no one can beat him. Current Naruto and current Sasuke more likely surpassed him in the 12 years time skip. You don't think 29-30 year old Naruto can pull the Bijuus out himself just because his 17 year old self was fighting for days, tired, and lacked the 1 tails and 8 tails chakra at the time so couldn't pull out their chakra without Bee and Gaara? RM Naruto could make vlone's for that and its a much stronger mode than BSM (which was about half or less when Naruto was fighting Obito, he was never at full power in BSM).
Also nothing says Madara couldn't be sealed by other means, that seal was to not only seal Madara with the Gedo Mazo but free the Bijuu's in the process as it did. Naruto with Yin Kurama and small amounts of the other Bijuu's was contending with Kaguya for a good while alone and he did the most damage to her, Madara wanted to get close to Hagoromo's and Kaguya's power. By the end of the war Naruto got Yang Kurama again so now he has full Kurama and thanks to his new arm he also have Hashirama's cells which gives stamina boost, powers up jutsus (KA is prime example), can give access to wood element and enhanced healing (only Madara shown having the actual healing ability while others only healed fast with it like Sasuke being revived by Kabuto).
Sasuke was new to his Rinnegan and look at all he did at 17, Madara and Sasuke both showed that they could use Rinnegan Jutsus with Susanoo (Madara used Deva path to pull down meteors and Sasuke looked to used Deva path to pull the CTs to him and Pretha path to pull out the Bijuu's chakra and mix it with his Susanoo). If Sasuke have the 6 paths then pulling out Madara soul is a way to kill him. Sasuke might be able to summon the Gedo out of Madara as Madara did to Obito but after he weakens Madara first. Sasuke can use the Sharingan to copy any jutsu's hand signs and the Rinnegan to use that jutsu element and be using dust release, lava release, boiling release, storm release, ect... It would be a closer fight in the Sasuke vs Madara fight no doubt but Sasuke still lived so he knows how strong Madara was and would improve his own skills and abilities over the years in case someone as strong or stronger comes.
Naruto was faster than Madara and Kaguya with only Yin Kurama so imagine Full Kurama. Plus his greatest strength is his huge chakra reserve he'll have in base+Hashirama cells and full Kurama's chakra so he can literally train with 100's of clones and improve his skills, jutsus, and knowledge faster than anyone. He makes 4 clones to go learn the basics of sword fight then goes off and train with 800 clones for a day and it's equal to training for 800 days; he would be skilled in kenjutsu and made it into his own style. He can do this with Taijutsu, weapon usage, sealing jutsus, chakra control, learning new jutsus from all elements, ect... All in those 12 years. Naruto without a doubt is the strongest by the end of the manga.
