I believe OP has got a point and while I agree with the majority that bitcoin remains decentralised in reality, the increasing adoption by institutions, buying on centralised exchanges, use as an investment and not P2P, etc., at least make it more centralised than it was intended to be..
What people do and how they decide to use BTC does not affect the network in itself, BTC should be stored in a self custodial wallet but if people decide to store their funds in a centralized exchange, then it is up to them and it does not change what the network is about. Institutional investors can buy as much BTC as they want, which can have an influence on the price of BTC, but it does not affect BTC's decentralization, as long as no central authority is in control and nodes are scattered all over the world, then the network remains decentralized.