Absolutely fantastic, yes! Roblox does allow you to make a game from scratch — even though the primitives are generally less primitive and the constraints much different — but my point is just that if you're wondering where the spirit of dj_mc_merlin's experiences went, two places where it can be found in spades are in the Roblox and Minecraft communities. (Source: Am a parent experiencing this vicariously via my kids.)
...with the caveat that once Roblox (the company) folds, all the games will be gone.
...and that since Roblox changes the code all the time, you have no idea whether your game will look the same in a year. Or even run.
So yes, while I understand what you mean by the spiritual successor.. Roblox takes everything that made Flash bad (i.e. dependence on a clunky binary that one day may not be supported), and makes it worse.
Source: worked in Roblox when a new shiny material pack was rolled out, which replaced existing materials in existing games, messing up the look of games where materials were used creatively.
True, and I guess Flash is a useful lesson in that sense, although no platforms I jammed with as a kid are around either. It makes me wonder if Godot apps written today will work on the web 20 years from now.
PS At least that was the case 9-10 years ago when I was involved.
Anyways, the client ui team definitely used Actionscript, which I had to read from time to time as a server-side dev.
A lot of games today use an HTML-powered UI library from Coherent Labs which can still be authored using Adobe Animate (i.e. what the Flash editor turned into): https://coherent-labs.com/