Are you sure you lost the planes in units on training missions only? I can't remeber ever losing a plane from a training unit, besides getting cought in an attack units. All OP loses i can remember happend on missions. Even the "break down in hordes" transport planes never get OP losses while training.
Well, it was partialy my fault in writing, in most cases it was only plane damaged (reading op report), and when plane was realy lost with pilot..well range was not set to 0, i forgot to change it, but othervise it was set to 100% trainig. It was in my traing area only, that why i got supicios, since i knowed i fly only traing missions here and i set all uniths here to traing only, well that was truth, but on some i forgot to change range.
Still i think option "keep at max posible numbers" will be usefull, not only to correct these types of mistakes like i did, but it will be usefull for patrol/tranport units too.
It will remove some nano-managment from game, leaving you more time for work with actual combat units.
And when we are on it, i think some coloring of fatigue on units will be nice to have, with red - this unit desperatly need rest, orange - nice idea to rest, green - ok condition, white - uni is very fresh :).
similiar for morale.