I totally agree. It is very unlikely that JAT will even consider ordering new 737s, when they have already concluded on future cooperation from Airbus Industrie.
The B737-300 is a very good plane, and should stay in the fleet for upto ten more years. Meanwhile the aircraft that are likely to replace the older Boeing and Douglas planes are likely to be a combination of A318s and A320s. In the long run, A319 should eventually replace the 737s. Certainly JAT is interested in a diversified fleet with the same operational characteristics, and as such its long term goal will not include both Airbus and Boeing for its mid-range fleet.
At this point, JAT does not need that many (eight) new mid-range aircraft; instead they need about four or five. This observation was also made by the Serbian ministry of transport. As a result, the assumption that a long range Airbus order is very possible. I am anticipating that JAT will make a well defined order by same-time next year.