If you consider scraping a threat then yes plain HTML might as well be giving up. The advantage of new reddit for that is quite clear: they can collect a bunch of data about your browser before deciding if you are a bot and if the rest of the page should load. The embedded recaptcha call in the screenshots is a pretty good hint. I suspect blocking trackers on new reddit will break as soon as the scrapers move over.

As for why they don’t just kill old reddit: a significant chunk of their active posters use it and are attached to it. So if they kill it entirely they will lose content. Posters are of course logged in so this change is less likely to affect them.