Being able to use someone else's passport....is not now and even more so in the future will never be a thing. But getting passports from more jurisdictions and choosing which to present- that is and will be even more of a thing.
Regarding your second point, there's a couple of dozens of countries that specifically require you to denounce your other citizenship to gain theirs, as well as those whose citizenship is lost automatically if you become a citizen of some other country.
I do see the cases of multiple citizenships (and therefore multiple passports) increasing in the future, but I wouldn't hold my breath for it to become a common occurrence within our lifespans.
Don't disagree. Could even see that exclusivity strengthening, a global movement towards national allegiance and homogeneity. Would be a terrible tragedy.