Watching this thread over several days (and being very familiar with all the aeroplane types mentioned - I used to have a share in a Warrior after doing my SEP PPL in a C152, and later did my EASA CPL in an Arrow) I took a while to form an opinion about this.
On the whole, I think that the PA28 is a good touring aeroplane, but a poor training aeroplane. The reason I don't like it as a trainer is that it is too undemanding of the pilot.
What will switching from a Warrior to an Arrow do? It'll push up pilot workload as they manage the gear and variable pitch prop - but certainly not to an unacceptable degree. It'll not solve the fundamental problem that the aeroplane's handling is exceptionally docile - although the steeper gear down glide will be more representative of some other types and the extra management of gear and prop in the circuit will push up the pilot's ability to handle complexity a little. It'll certainly prepare the pilot better to fly Arrows or similar if long touring flights is their eventual ambition. On net, it'll probably be slightly the better training aeroplane.
So, with the proviso that in my opinion there are significantly better basic trainers than either (e.g. the C150 and PA38 Tomahawk) I think it sounds a perfectly sensible plan. Instructors like the more complex aeroplane as well, and the availability will probably be better than the cheaper Warriors - both of which are likely to improve the learning experience,
Basically, go for it.
G