The thing people want from trees is shading and general cooling of the environment. Small plants provide much less of that and the summers are increasingly hot.
First of all, I'm skeptical about the study that proves that people seeing three trees have better mental health. There are so many factors that it's hard to separate one. A solid study would compare families living in the same building, roughly at the same floor, and with similar parameters (family size, income, education, street noise, etc.). Comparisons from different buildings induce too many side factors. I think that collecting this sample would be very hard. I can't access the full-text behind the paywall, so I don't know their methodology, and their abstract is vague, so I fear the paper is meaningless.
Then do people really watch that much through their windows? I'd be surprised that having a glimpse of a few trees at home once a day could change anyone's life.
Even if trees did has a positive impact on mental health, I suppose inciting people to bike or walk (at least partly) to workplaces and stores would dwarf that impact, for mental and physical health.
Lastly, the 30% of tree cover seems arbitrary. For the same percentage, would covering every street with trees have the same impact as keeping trees inside parks? I think the goal to provide places where people go for a walk requires different solutions than the goal to reduce the heat in a concrete jungle.