An overly simplistic conclusion based on a pick-and-choose approach to the report. First let me quote from the summary just a few pages down: "In sum, estimates of the aggregate economic impact of climate change
are relatively small but with a large downside risk. Estimates of the
incremental damage per tonne of CO2 emitted vary by two orders of
magnitude, with the assumed discount rate the main driver of the
differences between estimates. The literature on the impact of climate
and climate change on economic growth and development has yet to
reach firm conclusions. There is agreement that climate change would
slow economic growth, by a little according to some studies and by a
lot according to other studies. Different economies will be affected
differently. Some studies suggest that climate change may trap more
people in poverty."
Note that the literature diverges in its estimation of the impact. Much of the report on the economic impact basically says "there's a lot we don't know about most of these things" because the event haven't taken place yet. But that doesn't mean we can't extrapolate. Economic forces push populations. And, the effects will differ by geography, which is the crucial point. If only coastal areas are affected, that will still incentivize people to make decisions that affect inland economies. And these are the risks that a high proportion of the world's population would face (from the very same report):
The key risks that follow, all of which are identified with high confidence, span sectors and regions. Each of these key risks
contributes to one or more reasons for concern [RFC].
i) Risk of death, injury, ill-health, or disrupted livelihoods in low-lying coastal zones and small island developing states and other small
islands, due to storm surges, coastal flooding, and sea level rise.37 [RFC 1-5]
ii) Risk of severe ill-health and disrupted livelihoods for large urban populations due to inland flooding in some regions.38 [RFC 2 and 3]
iii) Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services such as electricity,
water supply, and health and emergency services.39 [RFC 2-4]
iv) Risk of mortality and morbidity during periods of extreme heat, particularly for vulnerable urban populations and those working outdoors
in urban or rural areas.40 [RFC 2 and 3]
v) Risk of food insecurity and the breakdown of food systems linked to warming, drought, flooding, and precipitation variability and extremes,
particularly for poorer populations in urban and rural settings.41 [RFC 2-4]
vi) Risk of loss of rural livelihoods and income due to insufficient access to drinking and irrigation water and reduced agricultural productivity,
particularly for farmers and pastoralists with minimal capital in semi-arid regions.42 [RFC 2 and 3]
vii) Risk of loss of marine and coastal ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for coastal
livelihoods, especially for fishing communities in the tropics and the Arctic.43 [RFC 1, 2, and 4]
viii) Risk of loss of terrestrial and inland water ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for
livelihoods.44 [RFC 1, 3, and 4]
Many key risks constitute particular challenges for the least developed countries and vulnerable communities, given their limited ability to cope.
IF you were correct in your assertion that the economic cost of doing nothing is lower than the economic cost of doing something, the ETHICAL cost of doing nothing is immeasurable because we'd willingly be exposing current and future generations to all of the above higher risks.