Being not convinced is not an unreasonable position to hold, but it hardly counts as an argument that renders the premise a nonstarter.

I am pretty sure that one can estimate approximately how much information would be needed to simulate a minimal universe consistent with everthing that humanity currently thinks it has been able to verify - in fact, I have a vague memory that Scott Aaronson presented something like that in one of his blog posts.

Then there are the uncomfortable gaps in our physical knowledge - the incomaptibility of relativity and QM, our inability to test, and choose between, string theories: suppose they cannot be resolved? I am not proposing that this is likely, but if it were, it would not mean that the universe could not continue, whether simulated or not.

I have read a bunch of "this is obviously wrong" claims, and one frequently-occurring aspect is that of begging the question somewhere, by assuming that something or other in what we seem to know about the universe is necessarily so. To be fair, proponents of the simulation belief sometimes also do this.

Personally, I don't have an opinion as to whether I am in a simulation; I put Bostrom's paper away in the file containing metaphysical solipsism, p-zombie arguments, and ontological proofs of the existence of God, as evidence for the futility of trying to reason oneself to ultimate truths.