Thread - will it solve all of our HomeKit problems?
Thread is potentially going to be really great, and potentially significantly better than many of the options we have today. If it lives up to its potential it is going to increase reliability, speed, and ease of use. It is going to reduce the burden on our Wi-Fi networks, which were never designed for home automation's high device-count, chatty behavior, and desire of low latency performance.
However, a lot of people are confusing/mixing/assuming/forgetting what Thread is and what Thread isn't.
Thread is like Bluetooth and Wi-Fi, it is the wireless medium of communication.
Thread is not like HomeKit and Zigbee, which is the control protocol and language.
Thread is the standard that defines how a device's radio can connect to another device's radio to create a network that allows them to communicate with each other using some other application's commands (like HomeKit).
HomeKit is the standard and framework that defines the command language that makes the devices actually do something (over a network like Thread).
So, while Thread has great potential to increase the reliability and speed of our smart devices, and can potentially improve some of HomeKit's communication limitations (unresponsive devices, performance improvements, etc), while at the same time freeing up our wi-fi, it is not necessarily going to magically change the landscape of HomeKit devices.
In other words, Thread ≠ HomeKit protocol support or HomeKit certification.
Vendors will still have to chose to invest their time and resources to design their Thread devices to understand HomeKit protocols and go through the HomeKit certification process.
Most vendors are probably going to continue to do this via HomeKit Bridges, like they do today (Although, the devices will connect with themselves and that Bridge more reliably over a Thread network instead of over Bluetooth network).
So. Thread is great. If it lives up to its promise, it will result in more reliable (HomeKit) smart device connectivity. But will it result in more devices supporting HomeKit? Probably not, at least not anymore than HomeKit just picking up speed in general in the marketplace.
By the way, Thread is kind of like what Lutron Caseta does, except not proprietary. It is one of the reasons that Lutron Caseta devices are so reliable (and responsive). It runs on its own proprietary radio network that works really, really well and extends itself. If Thread is built as reliably as Lutron's radio protocol, those HomeKit devices will likely be just as reliable as Lutron Casetas (and also likely to require a bridge).
Finally, CHIP, (of which Thread is a component) might be closer to what a lot of people think Thread is. CHIP is a truly cross-platform smart home automation platform. But that is even further away than Thread as far as widespread support.
Thread is a network transport, similar to Bluetooth and Wi-Fi.
Thread is not a control language, like HomeKit.
HomeKit can run on Bluetooth, Wi-Fi, and Thread networks.
Network Technology Pros and Cons:
has a very large range
requires considerably more power than Bluetooth or Thread
generally speaking, Wi-Fi is more expensive to manufacture
relatively speaking, Wi-Fi has high latency --in other words sending an "on" command to a device and seeing that device turn on would always be slower over Wi-Fi than a protocol designed for low latency, like Thread
Wi-Fi networks are harder to create and mange
Bluetooth and Thread
Both require much less power than Wi-Fi
but have a much more limited range
devices create a mesh network with themselves, thus extending its range, which is a major advantage over Bluetooth.
additionally, the mesh network is configured automatically and can automatically adjust to devices being added and removed.
Thread devices also have a lower latency when compared to Bluetooth (low latency means that they respond very quickly to commands)
HomePods, HomePod minis, and AppleTVs can all act as hubs and can all extend Bluetooth, within their Bluetooth range.
HomePod minis also support Thread, and so can help create and extend a Thread network and range.
Just because a device supports Thread does not make it HomeKit compatible. Thread devices still need to be made HomeKit compatible, either natively or via a Bridge, similar to Hue or Lutron.