[JJ_SO]

Nonsense

Count the camera models and then rethink "overcrowded"... and from the L-mount alliance there's still the first model to hit the shelves. So don't count non-existing choices in.

And also, don't forget Sony's APS-C attempts.

Given the small differences in size and costs, I really want to know why going on with APS-C. Each FF can do it as well, if it's only the crop factor.

There is Leica in the L-mount alliance and they have bodies available today.
I'm talking about manufacturers. The 135 format is going to get overcrowded very fast.
Granted, I forgot about Sony APS-C but clearly Sony doesn't care about it, focusing exclusively on 135. The 135 format has many more players than in ML APS-C. This is fact.

In terms of cost, there is a huge difference between ML 135 and ML APS-C. 
For instance a Fuji X-T100 or an EOS-M50 can be had with a kit lens for 600 CHF. Where can I get a ML 135 for that price?
If one consider MFT, an Olympus E-M10 Mk II with kit lens can be had for 450 CHF.
Why do everyone think that the 135 format is the solution to everything?

Now you really should take a very deep breath, Florent. Currently there's one single L-mount FF body around, the Leica SL, which sets you back for 6450 CHF - body only. Then there are three APS-C Leicas, the CL is one of the most expensive (2500 CHF) APS-C bodies around. The kit lens (!) goes for 1700 CHF... that's one of the "cheapest" you can get for L-mount, a 90/2 APO would cost 5520.- CHF

In your post you mention other cheap APS-C or µ 4/3 models. I find that very bizarre that you mention the super expensive high end L-system with only one body available and judge the 135 ML market as "overcrowded".

At the time of film cameras there were manufacturers with a lot more models than we see today. And the "historic stems of 135 format" at least led to a huge selection of MF lenses, much more than there ever will be available for the in-between APS-C stuff which never was more than a niche product (Olympus Pen and a couple of half-format Yashicas in film era), simply because bigger sensors were to costly to make and sell. I agree, there's a lot of coincidence in the 24×36 mm format and I don't fully understand why it has become so popular, but I guess the developing machinery for the films were already existing, the cameras cheaper to manufacture than MF and the films came from perforated movie material - so available in a bigger and cheaper selection than MF. There was enough production power behind that format.

135 is not "the solution for everything" and I don't have the feeling anybody ever said so. But APS-C or µ 4/3 are also not a solution for everything - in fact, they fall rather short as soon as it comes to wide angle lenses, but shine in tele (which are not really available from Fuji...). So, it's just the right tools for the job, nothing more or less. I do not see that big diff between these formats, not even in lenses: A Fuji 16/1.4 sells for 888.- CHF, a Sigma 24/1.4 for 745.-. I could bring other examples to show it's not cheaper to make great small, yet fast lenses. If you can live with slow zooms, great. I can't for what I want to do, and I need high ISO regularly, I sometime order big prints and I don't do so to show off, so speak for yourself

An A1 print out of a Fuji or µ 4/3  is more likely disappointing. And I always can go to APS-C with each FX model.