[Brightcolours]

EOS M is going to die soon. That system just doesn't make any sense.

Why does it not make any sense?
After all, not everyone want or need a 135 format system (with large and very expensive lenses).
If Canon were to provides descent bodies and lenses for their EOS-M mount, the system would be quite attractive actually.
They are just too short sighted to see it (as Nikon it seems).

I think this segment is already occupied by oter APS-C or µ4/3 manufacturers. The only thing Canon could do was gving the users long teles - these lack in a Fuji system. But then, these teles are not cheap nor lightweight...

The longer lenses get, the less difference it makes in weight/price whether it is APS-C or FF. 
Canon has enough tele lenses, it just lacks lower priced ones (like Nikon has been introducing).

But most serious photographers do gravitate towards FF... So the incentive to make relatively expensive tele options seems to lay with the manufacturers who have no FF to offer (Olympus, Fuji).

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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?

You answered your own question.
APS-C public often just gets a "kit lens" or two. That is its main public. The more serious/professional/artistic crowd spends money on specialized lenses and FF, mainly.

So yeah, there still is a place for EOS M.. The APS-C public which gets a kit lens and 1 or 2 other lenses (an UWA zoom, or the "street photography" 22mm, or a 32mm f1.4 "fast normal". or 55-200mm).