Plasma and GNOME are two completely different projects made by completely different organizations made on completely different technologies with completely different philosophies. That would be like proposing that McDonald’s and Wendy’s merge.
Yes, open source development isn’t necessarily as efficient and doesn’t lend itself to as nice of UX/UI/etc, but that’s not the point. The point is the freedom. Do I wish, as a GNOME user, that GNOME had certain features that Plasma does? Yeah, but part of the reason I like GNOME is that they’re so stringent about what makes it into the DE that it makes for an infinitely more polished experience than Plasma. You can definitely approximate the GNOME workflow on Plasma well enough, and that’s the great thing about Plasma: you can do almost anything you want with it.
You’re not the first person to propose that open source projects merge, and you certainly won’t be the last, but freedom also implies that you work on what you want to, so let people work on what they want to!
BTW, there are certainly more DEs than just GNOME and Plasma. Maybe try Budgie! It’s like the default workflow of Plasma mixed with the simplicity of GNOME.
In short, the maintainers have made questionable decisions over the years, and the Arch Linux packages are held back by two weeks on Manjaro for… basically no reason.
If you want an out-of-the-box solution to Arch Linux, just use EndeavourOS.
Endeavour has plenty of “beginner” tools, including a kernel manager (literally called A Kernel Manager) and a friendly GUI Welcome app that helps you update your system and your mirrors.