I do hope that we can turn this off. It should be optional for sure. We don't all have the time to fully test this roll out for SP. And that's how the current plan appears. That we are doing the functional testing.
I also spent a number of hours working with Fluid Engine and find that it removed many of the things previously separated SP from other simple web platforms. I know SP as a feature rich, simple to use platform that would predictably produce reliable results, with enough flexibility to take it to the next level if you had more web skills, while still being a no brainer to hand off or recommend to clients with little to no tech skills to manage.
Not having to worry about the mobile experience was huge. That is no longer the case. It's time consuming and fussy to get consistent layouts and reproduce things as others have said. All in all it's way more fussy and time consuming to work with without any gains from my perspective as someone comfortable working with CSS.
Other platforms (WIX) do this far better and begs the question why not use WIX if this is the audience you are now targeting? And yes WIX can be time consuming tweaking all the layouts and fussing with the mobile experience too but they've been doing this kind of user interface for a lot longer and have it sorted out -far better than the current state of FE.
I liked SP because it was different from WIX. I like WIX for what they've done too -but I chose SP. I've known people in the SP community wondering if they should change to WP due to other challenges, now this FE rollout might just make their decisions easier. There were definite advantages SP had over WIX, that now appear gone, at least at the drag and drop level of development. Ugh. Please give us back the classic editor. Please consider keeping that as an option as well so we don't have to babysit the mobile experience on every page, with every edit. It's something I chose over WIX that I know my clients like. And yes I know WIX is not necessarily the only alternative to SP but I mention it here because they have a drag and drop editor, it's biggest thing that separated it from SP in my humble opinion. There are a tonne of alternatives out there -many very good web platforms to choose from. In the fairly fast, flexible (design and function), easy-to-use, reliable, mobile-first category SP had an edge.