That’s why I said support members. They can use support musicians and stick by their word that they’re not replacing Reita as a full member of the band, but practicality weighs out over sentimentality. If they want to continue as a band, they could use the help. It’s not right to throw all guitar duties on Uruha and expect him to not burn out.
I know they’re not Japanese but one of my other favorite bands is Mastodon and they’re going through something similar. They were solid for 20+ years until something unexpected happened (they never elaborated on this) and they had to remove their guitarist Brent Hinds from the group. There was bad blood between them until Brent got hit while on a motorcycle and died. He was the secret sauce of the group and one could say his guitar playing is what differentiated Mastodon from their competitors, but that didn’t stop the band from integrating a new guitarist almost right away! They’re still touring and going strong even after the loss of one of their long time contributors. So it can happen. The band just has to be willing to let it happen, like you said, and then the fans have to be willing to accept it. That’s why I think having support members is the way to go.