From what I remember from the old series, Toya allowed Yue to absorb his magical powers because both Yue and Kero started to feed off Sakura’s magic when she became the new owner of the Clow cards. At this point Yukito started to eat a lot and feel sleepy all the time — he was technically disappearing. So I think, because I don’t clearly remember, Yue still transform into Yukito because Toya didn’t want Yukito to dissapear, not to mention it would have made Sakura sad.
It can be assumed Yukito has always been Yue's disguise since in the original series Kero knows Yue has a disguise and even thinks it might be Mizuki sensei. However, Kero didn't know it was Yukito because he had never seen Yue's false form before. Like you said, the false form is mostly a power saving mode for them. They never needed to save power when they were with Clow Reed so they never needed to use it back in the day. I wouldn't think they needed to use them anymore once the original story ended but I guess it's more convenient to. Kero and Yue can't just walk around in the real forms and Sakura is p much still hiding them from her dad and brother though even her dad should know about them since he has half of Clow's magic power.
In regards to the Touya thing from the previous reply, Kero has his own base power that keeps him alive. He gets enough power to transform into his real form once Sakura increases her sun attribute magic by capturing the relevant cards such as Firey. In contrast, Yue gets his power solely from his master. Initially, Sakura doesn't have enough magical power to feed Yue (hell, she can't even made a Sakura card without being exhausted). Therefore, Yukito subconsciously does his best to eat a lot of food to stay alive. Touya notices that Sakura wont be able to become powerful enough in time and once he sees that Yukito is dying he gives all his magical power to Yue so he and Yukito can stay alive.
yue stays inside yukito since he and keroberos agreed that sakuras powers are still not enough to sustain their real form, so they use that fake form to maintain their powers.
I've just binge watched the anime and read the manga, just so I can find the answer to ONE question!
What happened to Toya and Yukito? I mean, Yukito is just Yues other form he goes into to hide/save energy, but now, since everythings over, why is he still inside yukito, must be hella boring, since he can see everything yukito does, you know what I mean??