You know, Linux already has a large amount of customization and layouts. It's far more mature than Fuchsia, so you don't have to wait to try something new. You can also already answer texts from a Linux PC, sync notifications and use your Android phone as a remote control for the PC via KDEConnect and it's easy to use.
What do you mean "actually understands the latest devices and actually connects them on an operating system level"? That's literally what an OS does. If it didn't understand and connect to them on an operating system level, they wouldn't work. Also, Fuchsia uses a microkernel, so it's actually slightly less connected to the hardware "on an operating system level" than a monolithic kernel like Linux or hybrid like Windows NT. No idea what Cortana has to do with continuity between hardware platforms.
I'm not sure exactly what Google plans to use Fuchsia for, but I'll take a long time for it to replace existing operating systems, if it replaces any.
The biggest problem with Android has nothing to do with the UI, but how unmaintainable older smartphones are. So many smartphone manufacturers just distribute drivers in the form of binary blobs instead of contributing drivers to the Linux kernel. That make it very difficult to upgrade to newer versions of Android (or Linux) without breaking support for some of the phone's hardware. This leads to there being many different versions of Android in the wild and none of them being well maintained. If your phone is more than a year old, you might not get any security updates and probably not in a timely manner if you do get any. Unless Fuchsia becomes the next Windows, I'm not sure Fuchsia will be any better in this area than Android. What Google would gain from moving to Fuchsia is total control over development of the kernel, which could be good or bad.