Amblikai, flying can be incredibly conservative and that is reflected in the advice you are getting. Let me try to give you some facts as I see them;
In microlight flying there are aircraft which serve as 3 axis trainers which are lighter than any of the ones you are looking at. The Eurostar for example has been used as a trainer for many years on unlicensed strips and the schools have not gone bust - they have thrived. The syllabus for a micro PPL is almost identical that for a "group a" ppl.
Most of the "modern" group a trainers in use in the UK are cleared under CS-VLA. That is up to 750kg. The good old C150 is in this weight bracket and has been around for quite some time...
The problem is not that an aircraft is built down to a weight (see Eurostar example above) but that it is built down to a cost. Some of the new aircraft have not done well and the kit built SC has a poor record.
Rod1