Hmm on my Pixel it prompts me every now and then with a popup, even though I don't want to enable it.
I never use text messages, I already use Telegram and WhatsApp, and I don't like that, in the rare case I need to send an SMS, I'm not sure if it uses my internet connection or not.
weird. I had to manually enable it for several members of my family because "they couldn't join group chats" and "received poor quality photos". none of them had a pixel though, so perhaps it's a pixel thing to force you
For me at least it would always pop up and ask me if I wanted to turn it on during the initial setup of the app. I guess now it'll skip that step and just immediately turn it on.