Hi there,
First, congratulations!
Now, what we really need to understand here is why are you having second thoughts?
Is this for health and safety concerns or something else?
If it is not for health/safety reasons, then you absolutely should go with MBB. MBB is still just as prestigious, valuable, lucrative, etc. as before. Nothing here has changed!
If you're worried about health, then I would reach out to them and ask about travel policies etc. to check. That being said, I don't know of anyone at MBB currently travelling and I would expect that to remain the case until Covid is under control. I would be shocked if this changed AND if you could not cite health concerns as a reason to continue WFH.