The Oak Walker still have -75% aggro. If you are at -90% and doing something alone, you should increase your aggro. Tanks also need a job. Decreasing more aggro would destroy the need of a tank in some was.
I, as a tank, never had a problem with warden pets aggro since it is at -75%
Kind regards
After the last update, the pet's damage has increased. Because of this, problems with aggro
Again. Any Tank can easy hold the aggro.
If you have problems doing something solo (in a MMO), use an other rune to increase your aggro.
MMO is for playing together and teamplay and you should/need to pay attention if your tank is lower equipped
Kind regards