This is a good thing, and it's just a shame that so many addons haven't been updated yet.
Though losing Electrolysis will defeat one of the main benefits of running Firefox 57...
however I've just ran nightly and although it says vimfx will work as a legacy plugin it's not working, so the deal's off. I shall not use my mouse!
Other than a couple that were useful but ultimately not that important, and a couple I can do without until they get updated eventually, I managed to find that most of my Legacy extensions do have updated versions out. Namely, Video DownloadHelper and Greasemonkey, both extensions I thought would die with Firefox 57 out. That said, I wouldn't have known if I hadn't looked at the add-ons site as near as I can tell, while they still say "Legacy" in my Add-ons tab, they do have separate releases on the add-ons site specifically for Firefox 57 which are incompatible with my current browser and I'll probably have to install them again when I do update.* Assuming that is the case, you might find a decent chunk of that 70% already ready and you just have to take some extra steps to get them working again.
Overall, I am optimistic, lots of people have lots of nice things to say about 57 and the extensions situation is not looking nearly the sort of bad I was first expecting. I'm sure I've had more painful wait periods for my extensions going back almost a decade ago as it seems I really am waiting on just one and that one is coming shortly. Even those extensions which are now unsupported actually seem to have decent replacements and I've switched them over already.
* Note, this portion is entirely speculative, refer back to my commentary on waiting till an updated NoScript comes out before I actually install Firefox 57. It's possible they would just update once I do upgrade to 57 rendering my subsequent speculation entirely wrong.
I'll give it another chance in a couple of weeks, and hope that there will be a bypass that will force my outdated addons to work.