Per https://en.wikipedia.org/wiki/List_of_circulating_fixed_exch... the only African country with a currency pegged to the dollar is Eritrea. Not really seeing it.
You said "countries that depeg". Pegging, in the context of cryptocurrencies and regular ones, means something specific: ensuring that the exchange rate between the 2 is fixed within a certain band. I don't believe that the US is threatened by whether other countries peg their currency to the dollar or not.
The dollar's use as reserve currency is a separate matter.