I think the problem here is that Alfa are not really regarded as a "prestige" car brand like their German counterparts yet they are trying to target some of their cars in that very market space / price range.
Anyone who has that sort of money to spend on a car is generally going to go for one of the other brands with a proven repuation and track record in that market space. The Alfa brand unfortunately still has a bit of a stigma attached to it from years gone by and is still perceived by many as an unreliable brand even if that's not really the case anymore. It's sometimes hard to shake off that kind of reputation. Just look at Skoda. They have a terrible past history from the late 1980's even though the cars they produce today under the VAG brand can't really be faulted, yet I know many people who won't touch them with a barge pole. Maybe it's just badge snobbery but I'm sure past reputation also plays a part.
The depreciation rate of Alfa models also seems to be much faster compared to some other brands and this is always something people will consider when buying a new car. Trying to sell an Alfa on can be problematic because as you say they're in a niche market space so you either have to be prepared to wait around until an "Alfa lover" comes along and pays what you want for the car, or you have to take a hit and accept less than the car is really worth just to sell it on.
I really can't see that many people shelling out €50k or more of their own money on a normal Alfa even if it is a sporty model. Obviously the 4c and 8c are in a completely different league and something quite special which is why people do spend their money on those particular models. Most likely you will find that any owners of the Giulia will either be running them as company cars or will have them on some kind of a lease agreement. There may of course be a few die hard Alfa fanatics who will buy one of their own but I bet there won't be huge numbers of those.
I think I will wait for 3 or 4 years until that €50k car ends up becoming €15-20k on the used car market. Maybe then I will think about possibly getting one

That's assuming driverless cars haven't taken over the world by then

I will of course be more than happy to be proven completely wrong in my theory when the Giulia starts appearing in dealer showrooms