Of course the operators of many of these depend on low capital costs (so may not be able or willing to buy a new aircraft), and some others may perhaps be able to upgauge to an ATR or other 50-seater, but with respect, I find it hard to accept the assertion that there is no demand for this aircraft size when so many remain in operation
I think that this is the key to the debate - and where Saab's analysts will be focusing their attention. All things being equal, there will be a demand for the replacement of 340s, 120s, 360s etc but price/funding will be critical. Most, although not all, markets for these aircraft are at the impoverished end of the spectrum so potential users will not be able to buy because they won't get the credit and therefore will need to lease. However, given that their operations are marginally profitable anyway while operating well written down older aircraft, they won't be able to pay much in the way of rental and still make the operation profitable - even allowing for the likely reduction in fuel burn and maintenance costs of a new aircraft. The level of rental which these carriers can afford to pay dictates the maximum value the manufacturer can realise for each unit - and it won't be much. When you add the fact that there is a minimum non-recurring cost threshhold associated with developing a brand-new twin-engined public transport passenger aircraft almost regardless of size, you can see that optimising it at less than 40 or even 50 seats is just loading the dice against you. Even if it were possible to restart 340 production today without incorporating any new bells or whistles, Saab would be unlikely generate lease rates and therefore values high enough to cover costs. And, of course, as this is pretty much a lease-only market which conventional lessors will be unlikely to touch, the manufacturer will, in effect, act as lessor just as Saab, Shorts, DHC did in the past - and that's no way to build a genuinely successful programme.
As mentioned in other posts, the only way the 340, 120, 360 replacement market is likely to be satisfied is for a competent design to emerge from a competent manufacturer in an emerging very low cost country where a competent aircraft could be built at a price which works for the manufacturer and at a rental which works for the market.