It’s just physics. Radio frequencies that overlap can cause interference.
But you could have what you described exactly the way you described: stick to one protocol, and most likely it would need to be Wi-Fi since you will probably also have other Wi-Fi devices, and you’re done. The new Matter Standard will likely make that even easier.
Or maybe two frequencies, WiFi and thread, but still one standard, Matter.
Back when smartthings was first designed the thought was that we could take all these different radios on all these different frequencies and somehow mix them all together in one box. But, you know, physics. 
Hardware is hard.
Matter is likely to make things much simpler for the typical consumer. But you won’t be mixing together lots of different protocols anymore.