That's a good strategy. You can do a lot of smart home automation with two separate networks, where one has Internet access and the other doesn't.

Here's a reminder, keep your old routers, having an extra can really come in handy.

Will wifi switches locked down to home network only still function?

I don't know. My washing machine beeps three times in increasing intervals, so it isn't that intrusive. The display shows me unique error codes that I can look up when someone happens. And I can set the machine to finish in a set amount of hours, so it will start just in time to be done when I'm back. All without WiFi

My home assistant tracks the power usage of my washing machine (via a Tasmota plug) and notifies me when the power goes under a threshold for a few minutes. Which happens at the end of the washing cycle.

WTF is a "UE" error?

User Error, as always /s

How old is your washing machine? All of the washing machines that I have had over the last 15 years have had unique error codes, a button combination to turn off the buzzer and a delayed end functionality. Just set the program, click on the clock icon and set the program end time.

wet clothes going mouldy in the washing machine all day.

Never heard such a thing. I recommend to get a German one. Seriously.

I do something very similar with my connected dishwasher and Home Assistant. It's way over-engineered due various limitations/odd design choices with the API and the machine itself), but I've got it setup to store the selected program when I press a button on a Hue Tap switch, and then it turns on and runs that program when our off-peak energy rate kicks in - which is better than working out how much to set on the delay timer each evening to start it in the right ballpark.

Of course I've also got it setup to announce the selected program, and that the machine is "armed" via Google Home when the button is pressed, and again each time the door is opened/closed to add new dishes. And it sends notifications to my phone when the program starts (mostly for debugging purposes) and ends.

Like I said, massively over-engineered but it was a fun little project.

I don't have a smart washing machine (yet) but I do have it plugged into a smart plug with an energy monitor. When the power usage drops to near zero for more than 2 minutes it sends a notification to tell me that the cycle is done.

Exactly. Everyone has a device that can tell them when the machine is done. It is the programmable alarm in everyone's mobile phone.

Only company that makes dumb TVs anymore is Scepter, but those TV are a dice roll cause in the same model 1 tv can have a great screen and 1 can have a horrible one.. and I've had the bad luck of the draw to get multiple bad screens that looked like shit.

So I gave up and bought a TLC.

I cracked it open and unplugged the wifi antenna though, cause you can bet your ass that bitch will never have a chance to phone home and report any and all network snooping.

Would a large monitor with a streaming box work?

And what's the actual danger of connecting it to wi-fi?

Not so much that hackers will ransom the washing machine, but that hackers can use IoT devices as a back-door to get into your home network and take over everything else too.

Will hackers seize control of my machine and ransom my ability to get clean underwear?

It’s more likely that they’d seize control of it and add it to their botnet. Which is exactly what it looks like happened here. There was a small package downloaded, then a large amount of outgoing data. That looks like a compromised IoT device being used for a botnet. Small incoming package to hack the device, then the device starts spamming some poor dude across the country as part of a DDOS, because he beat a script kiddie in a COD match and the script kiddie is salty about it.

My washing machine has wi-fi. I didn't buy it for that reason, but it just happens to. Using the app

So you did not just connect it to your home wifi, but you also allowed the vendor to connect it to their servers. Now the vendor knows the name of your WiFi and the password. Just to begin with. Next year maybe this vendor's website will get hacked and 20000 such wifi passwords go public in some darknet :-)

Using the app, I have some programme options that aren't possible to select using the hardware dials.

Who benefits? You may find it cool to have it in the app, but FIRST the vendor has saved some of their money by not building the needed dials and buttons for these functions. (Or did they give you that discount? ;-))

And maybe in 3 years from now, they don't feel like maintaining your app anymore. Are you going to shout "WARRANTY" at them?

I just can't see the big downside here (other than the fact that the machine is more complicated than it needs to be

Yes, that is a downside, too. Part of this 'smartness' could break and maybe even the whole thing stops working when these 'diagnostics' give false data.

Another huge point is: My washing machines so far have lasted between 8 - 15 years. But NEVER has any wifi-active device lived that long. Think about this difference, and who's the one who benefits from it?

When it starts spying on you, sending conversations, like many smart cars, it is just the beginning.