Oh, thank God.
Finally.
It's going to be a while before there are actual products on the market and the ecosystem matures, and even longer before legacy devices fade away, but long-term having all three major players on the same platform is pretty much guaranteed to make a huge difference in terms of number of products available for any individual one, and the security of all of the above since I assume Apple cares more than Amazon or Google about that.
This particular XKCD joke/example of "A new standard that nobody will support because all the old standards are still in use!" is
entirely different than this sort of homogenization, because this isn't really a case of competing standards--this is a case where the three major players in an industry all had independent systems doing exactly the same thing so device manufacturers were forced to support each one separately, and often skipped one (or more) entirely.
Coming up with a shared standard when there are three big players in an industry isn't developing a competing standard, it's developing a
standard, period. It's defacto, yes, but if you have one home automation standard backed by Amazon, Google, and Apple, there really isn't anything else--you either support that or you support nothing, and it's unquestionably going to be cheaper for device manufacturers than supporting Amazon, Google, and HomeKit independently.
You don't need to worry about it competing with the other three popular standards because the companies in charge of those other three standards will (presumably) replace them with the new one, and the other 11 in the hypothetical example don't exist because the vast majority of the market is ruled by those three gorillas.
The analogy here would be a lot more like cell networks, which did the exact same thing--by eventually consolidating around one defacto standard, and adding in modems that read a lot of different bands, the days of X phone doesn't work on Y carrier or in Z country are gone; the same iPhone works on any of the US carriers and virtually any carrier elsewhere in the world.