It is definitely a big concern.
If countries or powerful people accumulate too much power, it defeats the purpose of bitcoin. The real purpose was to empower poor and weaker sections of the society. But if powerful have most of the bitcoin, they will become powerful and weaker section will suffer more. There is an additional concern that rich might be accumulating with different identities.
Bitcoin doesn’t just discriminate on who gets to own bitcoin and buy some, the real purpose of bitcoin is to serve as decentralization, to close the idea of third party system in the financial system. It gives full ownership and control to the person that owns it, provides for you the privacy you deserve of your own funds. Firms and countries holding more bitcoin gives room for manipulation of the system, but they won’t last forever or cab they change the overall concept of the technology. So mass adoption massively through them is a welcome idea, as the effect of them holding more is just temporarily to the market.