Responsive ads are not the only option! You should be able to find a “switch back to text ads link” (albeit hidden). This is usually at the top or bottom of the ad writing screen. That said, I have found that when building a fresh campaign that link may not be present. In that case, save and publish without ads (or make one temporary rsa if it doesn’t let you save). Then navigate to the ads section, click the plus, and then responsive search ad. From there, you should see the link to switch at the top or bottom of the screen ?
With that said, you also should set up at least one responsive search ad when setting up new campaigns as I anticipate sometime soon Google will fully switch over.
I hope this helps!