Future for post-mount discs?

Hope sell post mount calipers compatible with Shimano hydraulic sti levers. They work well and look great.

I wouldn't worry about it.  You can use Shimano's post-mount MTB calipers with road levers.  I'll be upgrading Mrs Srchar's steed (which has post mounts) from Tiagra 4700 mechanical calipers to Deore M6000 hydraulic calipers operated by RS405 shifters next week.

I think no chance of no more post mount based on ubiquity of post mount on MTBs. However, over long term you may have to buy caliper separate from lever. Or maybe someone somewhere really will come up with a flat mount (caliper) to post mount (frame) adapter. (Certainly if manufacturers want to stop making post mount calipers they will need to come up with a solution before all of the mountain bikers descend on them with pitchforks.)

Certainly no need to worry about it. Post mount disc brakes aren't going to vanish. I don't think we'll see post mount disc brakes included in R8000 R9100 groupsets (where flat mount is now the standard), but I doubt were going to see Shimano discontinue post mount callipers anytime soon.

I honestly wouldn't worry about it.  I have an Enigma Ecroix which has post mount brakes, I hope to keep it for many years yet and having post mount brakes on it doesn't bother me in the slightest.  For a start, it'll always be easy to get new brake calipers as it's still the standard in the mountain bike industry so there will always be plenty of upgrades available.  Secondly, I have always liked the look of post mount brakes, I think it's that fact that flat mount brakes look like they have far more metal in them, to bring the pads out enough.  They look like big triangular lumps of metal and I've never thought they made a bike look that nice. 

And another thing to consider is that whatever bike you get, there will always be something that comes along pretty quick that superseeds what your bike has (just look at the big push with through axles).  So just get the bike and have some fun on it.