Your suggestion is not problematic per se, but it's not the one advanced in the article. There are some very significant differences.
First, if it's the people who have the rights and not the river, than they can waive their rights as well. It's quite possible to imagine people who prefer development to a pristine river - this was the choice of every developing country after all.
Second, would the people have standing if the developments in question do not affect them or affect them positively? (e.g. diverting the river downstream, or drying up some swamps to halt some malaria-like disease).
The result is the giving communities those legal rights won't do for the purposes of those advancing this doctrine -
many communities' political representatives will just waive the rights immediately, perhaps in exchange for some investment or another.
The purpose of this legal doctrine seems to be to stop developments even if the elected authorities agree to them, so they have to give the rights to something which can't waive them or trade them for something else.
Even if you could get one lawyer/rights group to agree a certain development is worth it, there would always be some other group ready to advance a claim in the "river's" name.