You're right, its strength and "robustness" does rely in its decentralized nature, but that's truly the question. What's the threshhold? 50%? 60%? What if more than 70% of the total supply are held in the vaults of trusted-third-parties?
Currently, these are the largest centralized entities that HODL Bitcoin in their vaults, it's probably growing,
https://bitcointreasuries.net/?maximized=treemapIt could con tinue to get worse after legacy asset managers like BlackRock start buying and HODLing Bitcoin in their vaults too.
Those third-party services will cause more problems for people who use those platforms to store their bitcoins.
Will it affect people who are more knowledgeable, experienced and careful to store bitcoins in their non custodial wallets?
Short term, they can see losses on screen, paper losses when market is dumped by fiascos from third-party companies like FTX, Terra. But if they can hold, don't trade, don't use leverage, they will not have forced liquidation and they will be fine.
Long term, they will get benefit as people who lost bitcoins, and bitcoins lost forever including from third-party fiascos will be gift for everyone, donation to everyone.