It's easy to make a mess in many things, and it's true Flash has it's share due to it's sheer ability to create beginners in the realm of digital experiences.
We obviously also cannot support any of Flash Player's DRM features, not for any particular technical reason, but because we (and likely Adobe) are legally restricted from shipping a pure-web decoder for that DRM. We probably could proxy to an Adobe-provided EME plugin, but that would require cooperation with Adobe and major browser vendors for a 0.01%er preservation use case. We also cannot proxy early Flash video as no browser appears to expose native/hardware H.263 or VP6 decoding, so we'll have to ship software codecs that will probably perform like hot garbage on HD video. Fortunately, I don't think there's a lot of SWFs or FLVs with HD video in those codecs anyway.
Flash also allows you to enumerate device fonts, which used to be possible on the Web but isn't anymore. I don't think anyone will miss it.
However, the web sandbox is so large and all-encompassing nowadays that all of the above seems like small caveats. The vast majority of Flash content that people actually want to use is either standalone movies (Newgrounds, Albino Blacksheep, other portals) or static websites (e.g. Homestar Runner, Homestuck). The big hurdles for that are AVM2, dynamic text & inputs, and Stage3D. All of those are almost-perfectly enclosed within the web sandbox. Most of the sandbox-prohibited stuff is necessitated by websites which likely either have executed a transition plan to web standards (and thus won't be using Ruffle) or are willing to put up with whatever extra wrappers or steps we require to make these old APIs work.
How about flash games? I bet much more time was (and is) spend on flash games, than movies or static websites alone.
But games are performance critical.
[1]: https://www.adobe.com/devnet/articles/crossdomain_policy_fil...
Sure, someone has to run that proxy, but I'm sure flash games sites wouldn't have trouble doing so.
It's kind of a moot point, though; I doubt that the networking servers that these games depended on are still online. That's a whole separate preservation problem.
Alternatively, figure out a way to ship remote dependencies alongside the swf file, and define a patch format that allows replacing network requests.
All of this is more work to do, but all of it is also workable solutions.
There is a parallel universe somewhere where Javascript was designed as a Turing incomplete basic interactivity template and now node runs ECMAFlash2019
At the same time, I don't want to be dogmatic about anything, those are just general tendencies. Emacs for example is a beautiful example of an almost opposite design. The software is like a computing instrument that the user has to first learn how to play, but the more effort they put in to it, (up to a point at least, and depending on what they want to do) the more they get out of it. Ultimately it depends on what you're trying to accomplish and who your users are.
I've watched a few products "grow" using this method then project managers be surprised when down the line it's all held together with duct tape and chewing gum and a "simple" change is a week of work. There have been a number of times in my past where I have purposefully not suggested or the team decided not to bring up an "option" because we knew it would be abused.
He goes into some of the reasons it's a bad phrase and some of the problems behind it. I'd recommend the talk.
These are insanely hard problems yet they are solved because someone asked and someone tried.
For that you need server snapshot, that should be easy - just require it (maybe with source) to be provided with each application that use network. But some servers aggregate data from several users.
That's easy too - require it to be possible to deploy by user on some generalized network architecture.
We have same conversation again and again - The Right to Repair, Internet Archive, Open Source. Flash was part of that rhetoric too - that's why it is so polarized. Brilliant tool, so much creativity and yet closed source, poorly supported, bug ridden. It was a public stance - if Flash become standard it would be new IE once again.