I agree. In many cases 1.6mm is too thick and not needed for miniaturized projects where every 0.1mm counts, e.g. also for fitting some connectors. In such cases the higher stiffness if 1.6mm is also not needed.
My need for 1.0mm is every second project, so it is high priority for me. I solve it by choosing other providers - and accepting that it takes longer and has other drawbacks.
My need for 0.6mm is rare (about 1 out of 30 projects) so it has lower priority.
Even 4 Layers in 1.0mm would also be useful for some projects.
If there is a well defined layer stack (there aren’t many options for 4 layers 1.0mm) it is even possible to have impedance matched traces for high frequency applications. Rule of thumb is that required width is roughly the same as distance to ground plane to get ~50Ω.
If you only have 1.6mm with 2 layers you get trace widths wider much than your connectors (think of a Micro-USB or an USB-C socket)… Even 4 layers in 1.6mm give ca. 0.6mm wide traces. If AISLER could provide 1.0mm with 0.3mm depth of GND inner layer you have well handleable 0.3mm traces (exact calculations can be done with impedance calculators).
Since I am usually doing prototyping and have an EMS do mass production, AISLER is a good choice if the prototypes can be done in 1.6mm.
Therefore I am asking AISLER for years in every feedback for 1.0mm thickness in up to 4 layers and will continue until they eventually offer it 
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