Is there still a need to have defensive Pokemon?

No need for both Off & Def.

Atm probably no. But things can change quickly.

I continue my strategy for gym-destroying team lvl30 and def-team max lvl.

Until AI learns to dodge, I never believe there is any defensive move. Have u heard anyone who defends himself without dodging? There is no difference what to put in gym and thus might as well save all the stardusts for attack.

You never know how the game can change so you should keep your options as open as possible.

I can already destroy any gym with ease (just like most people). I feel like I don't need to improve my offensive team much further. I only have couple Pokémon with specific movesets that I'm actively hunting. For me this practically means that the game is extremely easy already and I can just keep hunting specific Pokémon to satisfy my OCD. I don't really care if I don't specifically NEED a Slowbro with a certain defensive moveset, I just want to have it because it's a tier 1.5 defender and only one that I don't have out of tiers 1 and 1.5 with perfect defensive moveset. It might only be my 15th best defender, but that doesn't really matter to me, I just want to have "a complete" team for both offense and defense.

The amount I fight gyms nowadays limits to 2-3 gyms a day so I could always use those 3 same Pokémon to defend the gyms but that's not how I like to play this game. You could also argue that you don't really need offensive Pokémon either because attacking is so easy anyway. My goal since I started playing was to get a perfect (well not necessarily perfect IV but high IV anyway) defensive team and offensive team. That's probably the only thing that keeps me interested in the game.

TLDR; There's no need for anything, attacking is easy and defending is impossible. Just play the game the way you feel most comfortable with. Playing optimally in terms of decisions versus playing terribly makes very little to no difference at all in a game with low skill cap (other than feeling better about yourself).

We've no idea what changes are around the corner. A high level defender will still go to the top of a gym and take that little bit longer to dispose of even if it's a matter of minutes.

The sensible move is probably to invest in dual use Pokemon e.g. vaperons with aqua tail.

Personally I'm investing in attackers that I enjoy using e.g. maxing out a Jolteon - not efficient but fun...