Defense tree will only be used on tanks still
Riot is trying to make the defense tree more accessible but the main problem is that the offense tree gives way too much multiplicative damage past 9 points.
In previous seasons, the main damage scaling was the %penetration. In seasons 2 and 3, you could get either 10% or 8% armor or magic penetration for 1 point. This allowed non tanks to go into the defense tree and still have relevant damage late game.
Now, the %penetration is way further down than 9 points, and you have dangerous game + spell/bladeweaving and the 21st point. This means that any champion who wants to do any relevant damage late game will go into the offense tree.
Basically bruisers will still take 21 points in offense because they want to deal relevant damage, meaning only full tanks will go deep into the defense tree.
If Riot wants more champions to go into the defense tree then they should make %penetration accessible for 9 points.
tl;dr offense tree gives wayyy too much damage to passup on for 90% of champs
catch me? tenacity and slow resist :P