I don't expect my light switches to stop working if the parent company gets acquired or shuts down

That's the simple case, where you are only using devices from a single vendor. If you - god forbid - own devices from two different companies and try to connect them in any meaningful way, your setup might suddenly stop working as soon as one of the following happens:

- Company A shuts down.

- Company B shuts down.

- The hub service that you used to connect Product A and Product B shuts down.

- Any one of the involved companies have a dispute with any other of the involved companies and cease cooperation/demand that their products not be used together until contracts are re-negotiated.

- Any of the companies makes a breaking API change and the other's don't have the time/money/motivation to update their software accordingly.