Firstly, the fertilisation effect is clearly a thing, so in that at least there is a discussion here. However:
1. The Fertilisation effect (FE) is insufficient to compensate for increased human CO2 emissions, although it may have served to slow the rate of growth somewhat:
https://newscenter.lbl.gov/2016/11/08/atmospheric-co2-pause/ . If emissions continue to grow, then the FE will likely not be fast enough to keep pace, returning to the original problem:
https://doi.org/10.1126%2Fscience.370.6522.1286-e .
2. Large-scale deforestation obviously acts against the FE, further reducing any potential benefits:
https://www.researchgate.net/publication/5798678_Climate_Change_Deforestation_and_the_Fate_of_the_Amazon
3. Increasing CO2 levels can have a deleterious effect on concentration of other minerals in plants, which impacts human nutrition:
https://elifesciences.org/articles/02245 ;
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4810679
4. Whilst the average rate of increase in temperature is indeed "only" 1/6 of a degree per decade, or thereabouts, this rate is also increasing:
https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature .
5. It is often said that the target is to limit global warming to 1.5 degrees, which has to be understood as being compared to pre-industrial temperatures. In that context, we have already reached a change of about +1 degree. With no change in policy towards climate, we are likely to reach a further degree of warming, with various unpleasant impacts as described in, e.g,
https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15_SPM_version_report_LR.pdf
6. It's also important to understand that any "doomsday" predictions, at least when made by experts rather than by politicians, refer to a point beyond which long-term changes are essentially "frozen in" to the system. If, for example, we reached carbon neutral tomorrow (ie, no further increase in CO2 levels), then many of the impacts of Climate Change, such as sea level rise, etc, would still continue for some decades before stabilising.
7. Likewise, many of the effects will operate on (relative to politicians and humans) fairly slow timescales. Current projections for Sea Level Rise, for example, are in the region of half a metre by 2100 (+/- 25cm), which is presumably a much slower rate than many people might imagine. This is, however, beside the point. If, say, the Maldives becomes uninhabitable in 100 years, as opposed to next week, that still matters and there's still the potential to do something about it.