Although an SS-ranked item was impressive, Izroth felt that Nature's Preservation alone was not worth being a hidden reward for this trial dungeon. However, that fact obviously changed when you took into account that it was possible to take away all the treasures within the trial dungeon.
There was no shortage of mythical and ancient quality materials in Franz's garden. Izroth even saw a few pseudo-divine materials during the time he spent gathering ingredients for the pills he crafted.
Nevertheless, Franz Winsor was willing to hand it all over to him not for the fact that she considered him to be her successor, but rather there was a request she wished to make.