You are forgetting something too. Items like life orbs and choice band have their drawbacks, e.g. taking away health, locking you into a move. Pokemon that dig-- excuse me, megavolve are confirmed to having their stats increased, which makes up for the inability to hold items while also having no other drawbacks. Even if the total BST do not increase it won't matter much because Pokemon like Alakazam if it were to megavolve, will probably have less atk more sp. atk and/or speed. Plus with new ability like mold breaker and magic bounce and adaptability, I will be more than willing to give up on locking myself into one move (choice items) or taking damage every time I attack (life orb). Not to mention, gaining new type such as dragon can make a whole lot of differences. Also, there may be a separate item slot for mega stones so you never know. Do I think digivolved mons will be stronger than their pre-evolved forms? Of course! What's the purpose of digivolution if not for getting stronger?!