The screen locker just removed custom themes a few weeks back out of concerns that the (executable code) theme can capture passwords and potentially do malicious things with it.
The challenge with the screen locker is that it’s a single window that draws both background and password dialog into the same window. Whereas xscreensaver is a separate window that would have to replace the background, while at the same time it needs to be prevented from capturing user input. From a technical perspective, embedding the contents of some other program’s window is more involved than reusing Plasma’s wallpaper plugins, which is something that can be done already.
I’m sure it’s doable in some way, but it’s not as easy as simply flipping a switch, and if we get it wrong on the input handling side then that’s a security hole right there. That’s not a patch you’ll get done in 2-3 days of work, that’s a serious chunk of time that someone will have to invest, at the expense of other development efforts to fix bugs or improve productivity or get apps & games running more smoothly. A developer will ask themselves if Plasma wallpaper plugins aren’t perhaps already a good enough replacement and we just need more of those.
Maybe the answer is yes, it’s worth it, but so far no one has stepped up to invest either their time or money in getting this development done. This may change in the future if there’s a big enough movement.