I kind of agree with the GP, mostly because I feel like if we could focus the efforts of the various DE communities on a single project then we could finally get a Linux DE that has as much polish as Windows and MacOS.
Why do you think that all these different DEs exists? Because all those people that currently work on all those different DEs wouldn't be able to agree on anything if they were all working on the same DE. Lets also not forget about the mythical man month.
Probably just a pipe dream though.