I see no reason for GM bond holders to accept any offer to convert their bonds into stock.
GM can't force them to convert, and bond holders will do far better than this in bankruptcy - unless GM is willing to substantially dilute current shareholders into a tiny percentage of the shares.
I doubt the current offer puts current GM shareholders into this much pain. In bankruptcy, the current shareholders will receive nothing. It's worth giving the current shareholders a tiny sliver to avoid the bankruptcy filing, but beyond that the bond holders have no incentive.