Replacing Control Arm Bushing on 2004 Toyota Sienna

I have been told by the local Toyota dealership that the left front control arm bushing needs to be replaced in my 2004 Toyota Sienna. They say that the bushing has corroded some. They said they noticed the problem when test driving it and pressing down on the brakes. They are saying that the engine and transmission has to be removed for them to replace the control arm bushing and the quote of $1600 includes the part and labor of 15 hours. Do they have to remove the engine and transmission to replace this part. Is this an honest cost estimate?

I'm worried that I'll have more problems with the engine and transmission because they have been removed and put back into the van. The van has close to 165,000 miles on it. My biggest worry though is what will happen while driving if I don't get this replaced. What are the risks of not getting this fixed?

Thanks,
Paul