This is not an issue because how much BTC you wrap in a single DBC-wallet is up to you, which makes this an easily manageable risk: you can create multiple wrapped BTC in different DBCs to handle larger quantities with less risk, similar to spreading BTC holdings across multiple addresses.
It is of course valid to ask what is the risk of a Sybil attack, but I think it will be small enough that you should be able to hold large value in a single DBC. What is an acceptable risk for a given value will be case by case, so what is needed is not to rule this out, but to have confidence in the level of risk.
Safe Network can be expected to achieve very good resistance to Sybil attack and we have been promised the maths behind this. This will be tested theoretically, the code audited, demonstrated practically (e.g. with hacker challenges) and tested over time with real world operation. Over time we’ll be able to say with increasing confidence what kind of risk is involved and the scale of value which can be secured, and whether or not there are use cases which would benefit from enhanced measures, such as @Antifragile suggests.