I don't think many adventure game fans would agree with you.
There are both linear adventure games and ones that are more freewheeling (such as The Last Express). This is also true of RPGs. That aspect doesn't define the genre unless you radically redefine everything.
Adventure games have no combat, or some combat in some cases, but it's not the emphasis of the game. They never have a fleshed-out combat system with statistics, equipment, spells, experience, etc. Adventure games typically have no use for statistics such as attack power, defense, agility, HP, MP, and so on, or the raising of such. Adventure games rarely have boss fights. Adventure games are driven by puzzle-solving, exploration, character interaction (dialog), and/or item manipulation.
Bottom line, RPG combat = RPG, IMO. Final Fantasy has more in common with Diablo than it does with Sam & Max.