why to spent money on a RF 45mm 1.2 STM lens with similar optical performance but weaker built quality
Ditching the adapter, IMHO of course, is worth enough for spending the money; also, I don't know the used price of the EF 1.2 L but I guess is on pair or more expensive then the new RF 45 1.2 so there's also some possible saving. If the thing is good enough, I'll gladly ditch my 40 Art and hopefully get 50/100€ extra in return (but same price swap would be ok, too), without the hassle of the adapter.
Then, build quality in 2025 is good enough even on cheapo chinese lenses, let alone a first party f1.2 lens, which is also going to be serviced for the next decade or two, while if the EF L is not going to be serviced for that long (if even still serviced today), and could end up becoming a fancy paperweight.
That's why I immediately swapped the EF 24-70 L II with the RF 28-70 STM as soon as I saw that quality was same or better; new lens, two years guarantee, no service problems for at least a decade, no adapter, shorter, lighter, stabilised. The new RF 45 would likely be the same, if quality is what we all hope and expect.