If you don't have any cycles to spare, you can upgrade to an MCU that does have cycles to spare for less than $0.50 in small batches, and an even smaller price delta for larger batches.
Any device that doesn't use cryptography isn't using it because the manager has specifically de-prioritized it. If you can't afford the $0.50 per device, you probably can't afford the dev that knows his way around cryptography either.
Well, no. If you can do 1 AES block per second, that's a throughput of a blazing fast 16 bytes per second.
I know that's a pathological example, but I do understand your point - a typical workload on an MCU won't have to do much more than encrypt a few kilobytes per second for sending some telemetry back to a server. In that case, sure: ChaCha20-Poly1305 and your job is done.
However, what about streaming megabytes per second, such as an uncompressed video stream? In that case, lightweight crypto may start to make sense.
An uncompressed video stream at 240p, 24 frames per second is 60 mbps, not really something an IoT device can handle. And if the video is compressed, decompression is going to be significantly more expensive than AES - adding encryption is not a meaningful computational overhead.
Eg a small edge gateway could be doing the VPN, while the end device is decoding the video.
Modern encrypted streaming uses pre-existing compressed video where the packets are encrypted on the way to you by the streaming server. It would have to uniquely encrypt the data being sent to every single user hitting that server. So it's not just a one and done type of thing. It is every bit of data for every user. So that scales to a lot of CPU on the server side to do the encryption. Yes, on the receiving side while your device is only dealing with the one single stream, more CPU cycles will be spent decompressing the video compared to decrypting. But again, that's only have of the encrypt/decrypt cycle
>1 operation per second would refer to cryptographic signatures. If you are doing Chacha, the speeds are more like 1 mbps. AES is probably closer to 400 kbps.
It sounds to me like you, sir or madame, have not worked with truly tiny MCUs. :-)
But yes, there are inexpensive MCUs where you can do quite a bit of crypto in software at decent speeds.
The monetary cost is most likely not the problem. Tacking on significant additional work is bound to consume more power and generate heat. Tiny devices often have thermal and power limits to consider.