I don't know specifically about HomeKit, no experience with it, but many of the underlying standards are "old". E.g. Zigbee is 20+ years old and was standardized in 2003.

Most of these systems today also let you put in place one hub per standard you care about, and then just update devices, and you can mix and match. Some, like Zigbee, are standards supported by multiple manufacturers. The radiator thermostat interface I rely on is standard for UK radiators at least, and is physical and pre-dates electronic thermostats by decades - the smart thermostats physically press down on a switch on the radiators; to replace them with another standard - or a manual one - you just twist the old one off and twist a new one one, done in ~2 minutes.

They similarly communicate with a hub, and an optional central boiler thermostat. The boiler thermostat is the only part that took any time to figure out (but there are plenty of installer services available), and the thermostat interface for boilers in the UK at least is built so that anyone can build replacements; you just need something that can switch a current.

Keep in mind that the only thing that is new here is the network enabled smart control - many of the interfaces are already really old and established.

I have ~20 Zigbee lightbulbs. I also have some LightwaveRF switches. As I refurbish and/or as the Zigbee bulbs die, I'll slowly replace the Zigbee lightbulbs with Lightwave switches as they are more convenient. But the difference is small enough that I've not bothered going over the house to replace them all. If either one of them become hard or expensive to come by, I'll just at that point add a hub that can bridge between a new standard and whatever device I use to interact with them. Setting up the Lightwave hub took me less than half an hour. If I have to invest half an hour to add a new type of hub every 10-20 years, I'm fine with that.

In this case, if Lightwave "dies" entirely and becomes impossible to support, I'll lose the smart functionality of those switches, but nothing else is hard-wired (the Zigbee setup is all mesh between the bulbs themselves, and a hub built into one of my Alexas, and they too require no extra wiring that a normal switch doesn't, and fall back to working like a slightly fancy normal switch if the network is unavailable, and someone unaware of Lightwave wouldn't know anything was broken. And that's what I'd face while transitioning to something new.