I entered the S system when it was new and paid a lot of money for all those lenses, same with the cl, both still work but both are old systems. New products from Leica in 2024 were rebaged sigma lenses for double price. I trust more in the sl system and M system than another new s system. But Hey, maybe I will become week anyways, owning all thisM!sl and s glass.
But for now I prefer to have a modern camera and lenses in my hands and shoot it Instead for waiting for something which might come one day. Still I love and use many of the Leica products.
Well your're in the best possible position, literally.
Essentially you have no glass upfront cost and you'll benefit from the ultimate Leica camera experience - ie best EVF, highest res, best colour, etc.
I sold my SL in anticipation of the S4 and will most likely stick with M12M and S4 - S4 covering all colour bodies; the S4 will not be THAAT much bigger compared to the SL which makes the SL3 a bit obsolete in that regard. The sensor will only be 44x33 ish, which doesn't lead to a humongous body difference if they take SL3 tech, improve EVF etc.
So the things to keep are:
+ M: Summiluxes, Noctiluxes, APO 35, 50
+ SL: APO SL glass
+ S: All glass
Then bit by bit you can buy a new S4 optic, but IMHO any Leica aficionado with spare cash will jump on the S4, especially because its highly likely that it'll be the only body with autofocus M glass + lens specific in camera corrections ...
If you own Leica bodies, you can just upgrade for a tiny bit bigger body and cross-finance it by dumping all colour bodies, like the SL3 which should still be like 4.5k by the time the S4 hits.
The APO SL lenses will naturally be smaller than the S4 APO lenses, so there's a good reason to keep them still.