Exactly how?
Yeah, so, you are right about decentralization being important, cant argue there. But the idea that a 51% attack means someone literally "owns" most of the Bitcoin? Nah, thats not it. What actually happens is a group ends up controlling more than half of the network computing muscle (the hashrate, if you wanna get nerdy about it). Owning 10.71 million BTC or whatever is totally unrelated to pulling off a 51% attack. The danger comes from controlling the network, not hoarding coins. Thats a big difference.
Economic holders might not be able to participate in network consensus directly. But they can "bribe" developers or influence the direction of the project with their wealth. Money talks, right? Now, imagine those same economic holders (institutional investment firms, governments, banks, etc) also running nodes and miners of their own. Bitcoin would be nothing but compromised. Assuming they manage to control a large portion of the network's hashrate.
For what I know, all "cards" are on the table. It would've been better if BTC were a multi-algo PoW cryptocurrency like DigiByte (DGB). This would've made it harder for centralizing forces to gather 51% of the hashrate under one algorithm. I trust that developers will come up with a solution that prioritizes decentralization above all else. Therefore, there should be nothing to worry about.