But i said that PoE 1 had extremely weaker spells and hence had to be changed to per encounter on Deadfire, because more powerful a spell is, more limited his usage should be.

it was changed to per-encounter, because Obsidian decided to do so. If they wanted spells to be stronger they would have done so. Spells are still really effective in both PoEs. PoE2 might have made spellcasters a bit too good IMO.
They already experimented with per-encounter design in PoE1 expansions. There is no point to rest system, in a game which doesn’t have structure to pace rests. It was always a problem in Infinity games. Per-encounter makes power available to player constant and therefore allows for better encounter design.