>But today, you can be stressed out about Trump or COVID-19
Stress has nothing to do with it. I can agree that stress rarely has application in our modern day lives. You can be a stress-free negative thinker and also it is possible to be a stressed positive thinker. (positive thinkers fail more, it is possible for that to have an impact on people)
>becomes grief - denial, anger, bargaining, depression
Again, none of these are necessities to be a negative thinker.
>Most things to be negative about are not actionable.
That is correct. It is undeniable that negative people are not very effective in our world. Even further - all negative philosophies and religions (Buddhism, stoicism, western pessimism, etc.) focus people on enlightenment and perfecting the person's inner world. A monk has zero interest in changing the world at scale.
However it seems to me that there are certain practicalities about negative thinking. You can be smart with it. If you take it at face value - negative thinking is kind of a superpower, it allows you to think what can possibly go wrong and frequently you more cautious than you had to be. You can use that to minimize risks and if successful - you can take actions, because the risk is no longer that high.
Most things to be negative about are not actionable. Maybe in the old days, you could be stressed out about a tiger in the area or an upcoming war and it'll literally save lives. But today, you can be stressed out about Trump or COVID-19 and it'll probably change nothing. In fact, COVID-19 impact has been worst where people are most panicked about it.
There's also where it becomes grief - denial, anger, bargaining, depression. This is not realistic nor useful.
Optimism is a filter. You don't purposely blind yourself; you focus on what you can have a lot of impact on. You can turn the filter on and off (provided it wasn't installed stupidly in the first place).