Most of those small and medium hydropower plants generate near insignificant amounts of electricity, even if you damn all the non major rivers you can (and destroy all the surrounding ecosystems) it would be hardly noticeable.
My napkin math disagrees. Small/medium hydro (<100 MW) appears to account for ~1.7% of annual US grid energy output. If we replace that with fossil fuel generation and account for the cost of dam decommissioning, account for estimated benefit of ecosystem restoration in terms of quantified dollars willing to pay by benefiting citizen, and net annual CO2 cost I come up with an annual cost to society of removal of small/medium hydro dams between $1.7B/year on the low and $41.7B/year on the high end. Happy to show detailed assumptions and calculations if desired. But tl;dr fossil fuel generation is very, very bad and replacing even comparatively small amounts of carbon-neutral generation with it results in a significant societal net negative.