Way to miss the point
All these developers are scratching an itch. I have no insight why they are doing it and they have no obligation to meet my needs or demands.
To call it wasted effort is to attribute motive without evidence and I would suggest denigration. I consider myself lucky to be able to use what others have developed.
Case study Timothy Pearson (TDE) decided that KDE3 was where it was at and forked the code base. His time, his decision.
I quite liked Konqueror being a local file manager (everything is a file, after all) but despite the loss not enough to choose TDE over current KDE. I took a look, it was nostalgic but I'm tagging along as a mostly free rider with the KDE project. (I've also looked at a couple of the other desktops that openSUSE provides but KDE does it for me. My dislike of GNOME is entirely unevidence based.)
Of course all this was under my control (and responsibility) with no-one nagging me to do this or that. That's one of the plus points.
It's just how it is. I don't think diversity of desktop is a problem. The paid for edifice of commercial software might get so unwieldy that even their obscene marketing budgets might fail to stop the collapse (slight evidence that windows 11 might be in the space) leading to a significant search for an alternative but until then just be grateful for what you have got and stop worrying about the year of LOTD.
BTW on datelines KDE was a free evolution of Sun's CDE unfortunately although reassurances were in place at that time Qt was free as in beer not as in speech (now both) So the GNU project started GNOME.