Of course, what every manufacturer wants is for you to stick with only their devices. As pointed out, for the larger manufacturers at least, "open and interoperable" is the last thing they want.
Now, whether one manufacturer provides the functions needed/wanted is another issue. If manufacturer 1 offers A,B, and C, and manufacturer 2 offers A, C, and D, and you want A, B, and D then you're stuffed unless you can get two makes of kit to talk to each other.
Now, in some fields, there is enough recognition that interoperability isn't optional - then manufacturers (or more likely protocol groups or trade groups) will get to together to hold interoperability lab sessions. The manufacturers come along with their kit, try it with other kit, and then figure out what doesn't work and how to fix it.
This only happens when it's something that the public just won't tolerate "issues" and having "non compatible" kit means getting a crap reputation and being locked out of the market - and it also requires that no single player has enough of the market to dictate things.
IoT isn't one of those fields - whether it gets to be one in the future, well your guess is as good as mine, but I suspect it's going to be driven by a small number of "big names" working on the "work by my rules or not at all" attitude to smaller manufacturers.